diff --git a/DESCRIPTION b/DESCRIPTION
index 9b857d43..53f20b81 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,6 +1,6 @@
 Package: protti
 Title: Bottom-Up Proteomics and LiP-MS Quality Control and Data Analysis Tools
-Version: 0.9.1
+Version: 0.9.1.9000
 Authors@R: 
     c(person(given = "Jan-Philipp",
            family = "Quast",
diff --git a/NEWS.md b/NEWS.md
index f4e23d7e..fa529c28 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,6 +1,13 @@
+# protti 0.9.1.9000
+
+## Additional Changes
+
+* `assign_peptide_type` now takes the `start` argument, containing the start position of a peptide. If a protein does not have any peptide starting at position `1` and there is a peptide starting at position `2`, this peptide will be considered "tryptic" at the N-terminus. This is because the initial Methionine is likely missing due to processing for every copy of the protein and therefore position `2` is the true N-terminus.
+
 # protti 0.9.1
 
 ## Bug fixes
+
 * `try_query()` now correctly handles errors that don't return a response object. We also handle gzip decompression problems better since some databases compressed responses were not handled correctly. 
 
 # protti 0.9.0
diff --git a/R/assign_peptide_type.R b/R/assign_peptide_type.R
index a219d610..defe7360 100644
--- a/R/assign_peptide_type.R
+++ b/R/assign_peptide_type.R
@@ -24,7 +24,9 @@ peptide_type <- function(...) {
 #' peptide is located at the N- or C-terminus of a protein and fulfills the criterium to be
 #' fully-tryptic otherwise, it is also considered as fully-tryptic. Peptides that only fulfill the
 #' criterium on one terminus are semi-tryptic peptides. Lastly, peptides that are not fulfilling
-#' the criteria for both termini are non-tryptic peptides.
+#' the criteria for both termini are non-tryptic peptides. In addition, peptides that miss the initial
+#' Methionine of a protein are considered "tryptic" at that site if there is no other peptide
+#' starting at position 1 for that protein.
 #'
 #' @param data a data frame containing at least information about the preceding and C-terminal
 #' amino acids of peptides.
@@ -34,49 +36,90 @@ peptide_type <- function(...) {
 #' acid as one letter code.
 #' @param aa_after a character column in the \code{data} data frame that contains the following amino
 #' acid as one letter code.
+#' @param protein_id a character column in the \code{data} data frame that contains the protein
+#' accession numbers.
+#' @param start a numeric column in the \code{data} data frame that contains the start position of
+#' each peptide within the corresponding protein. This is used to check if the protein is consistently
+#' missing the initial Methionine, making peptides starting at position 2 "tryptic" on that site.
 #'
 #' @return A data frame that contains the input data and an additional column with the peptide
 #' type information.
 #' @import dplyr
 #' @importFrom magrittr %>%
 #' @importFrom rlang .data
+#' @importFrom stringr str_detect
 #' @export
 #'
 #' @examples
 #' data <- data.frame(
-#'   aa_before = c("K", "S", "T"),
-#'   last_aa = c("R", "K", "Y"),
-#'   aa_after = c("T", "R", "T")
+#'   aa_before = c("K", "M", "", "M", "S", "M", "-"),
+#'   last_aa = c("R", "K", "R", "R", "Y", "K", "K"),
+#'   aa_after = c("T", "R", "T", "R", "T", "R", "T"),
+#'   protein_id = c("P1", "P1", "P3", "P3", "P2", "P2", "P2"),
+#'   start = c(38, 2, 1, 2, 10, 2, 1)
 #' )
 #'
-#' assign_peptide_type(data, aa_before, last_aa, aa_after)
+#' assign_peptide_type(data, aa_before, last_aa, aa_after, protein_id, start)
 assign_peptide_type <- function(data,
                                 aa_before = aa_before,
                                 last_aa = last_aa,
-                                aa_after = aa_after) {
-  data %>%
-    dplyr::distinct({{ aa_before }}, {{ last_aa }}, {{ aa_after }}) %>%
-    dplyr::mutate(N_term_tryp = dplyr::if_else({{ aa_before }} == "" |
-      {{ aa_before }} == "K" |
-      {{ aa_before }} == "R",
-    TRUE,
-    FALSE
+                                aa_after = aa_after,
+                                protein_id = protein_id,
+                                start = start) {
+  # Check if there's any peptide starting at position 1 for each protein
+  start_summary <- data %>%
+    dplyr::group_by({{ protein_id }}) %>%
+    dplyr::summarize(has_start_1 = any({{ start }} == 1), .groups = "drop")
+
+  peptide_data <- data %>%
+    dplyr::distinct({{ aa_before }}, {{ last_aa }}, {{ aa_after }}, {{ protein_id }}, {{ start }}, .keep_all = TRUE) %>%
+    dplyr::left_join(start_summary, by = rlang::as_name(rlang::enquo(protein_id))) %>%
+    # Determine N-terminal trypticity
+    dplyr::mutate(N_term_tryp = dplyr::if_else(
+      !stringr::str_detect({{ aa_before }}, "[A-Y]") | {{ aa_before }} == "K" | {{ aa_before }} == "R",
+      TRUE,
+      FALSE
     )) %>%
-    dplyr::mutate(C_term_tryp = dplyr::if_else({{ last_aa }} == "K" |
-      {{ last_aa }} == "R" |
-      {{ aa_after }} == "",
-    TRUE,
-    FALSE
+    # Determine C-terminal trypticity
+    dplyr::mutate(C_term_tryp = dplyr::if_else(
+      {{ last_aa }} == "K" | {{ last_aa }} == "R" | !stringr::str_detect({{ aa_after }}, "[A-Y]"),
+      TRUE,
+      FALSE
     )) %>%
+    # Assign peptide type based on N-term and C-term trypticity
     dplyr::mutate(pep_type = dplyr::case_when(
-      .data$N_term_tryp + .data$C_term_tryp == 2 ~ "fully-tryptic",
-      .data$N_term_tryp + .data$C_term_tryp == 1 ~ "semi-tryptic",
-      .data$N_term_tryp + .data$C_term_tryp == 0 ~ "non-tryptic"
+      .data$N_term_tryp & .data$C_term_tryp ~ "fully-tryptic",
+      .data$N_term_tryp | .data$C_term_tryp ~ "semi-tryptic",
+      TRUE ~ "non-tryptic"
+    )) %>%
+    # Reassign semi-tryptic peptides at position 2 to fully-tryptic if no start == 1
+    dplyr::mutate(pep_type = dplyr::if_else(
+      .data$pep_type == "semi-tryptic" & {{ start }} == 2 & !.data$has_start_1 & .data$C_term_tryp,
+      "fully-tryptic",
+      .data$pep_type
+    )) %>%
+    # Reassign non-tryptic peptides at position 2 to semi-tryptic if no start == 1
+    dplyr::mutate(pep_type = dplyr::if_else(
+      .data$pep_type == "non-tryptic" & {{ start }} == 2 & !.data$has_start_1 & !.data$C_term_tryp,
+      "fully-tryptic",
+      .data$pep_type
     )) %>%
-    dplyr::select(-.data$N_term_tryp, -.data$C_term_tryp) %>%
-    dplyr::right_join(data, by = c(
-      rlang::as_name(rlang::enquo(aa_before)),
-      rlang::as_name(rlang::enquo(last_aa)),
-      rlang::as_name(rlang::enquo(aa_after))
-    ))
+    # Drop unnecessary columns
+    dplyr::select(-c("N_term_tryp", "C_term_tryp", "has_start_1"))
+
+  # Join back to original data to return the full result
+  result <- data %>%
+    dplyr::left_join(
+      peptide_data %>%
+        dplyr::select({{ aa_before }}, {{ last_aa }}, {{ aa_after }}, {{ protein_id }}, {{ start }}, "pep_type"),
+      by = c(
+        rlang::as_name(rlang::enquo(aa_before)),
+        rlang::as_name(rlang::enquo(last_aa)),
+        rlang::as_name(rlang::enquo(aa_after)),
+        rlang::as_name(rlang::enquo(protein_id)),
+        rlang::as_name(rlang::enquo(start))
+      )
+    )
+
+  return(result)
 }
diff --git a/R/qc_cvs.R b/R/qc_cvs.R
index 713303d3..3ffa1ff2 100644
--- a/R/qc_cvs.R
+++ b/R/qc_cvs.R
@@ -122,7 +122,7 @@ The function does not handle log2 transformed data.",
         dplyr::mutate({{ condition }} := forcats::fct_expand({{ condition }}, "combined")) %>%
         dplyr::mutate({{ condition }} := replace({{ condition }}, .data$type == "cv_combined", "combined")) %>%
         dplyr::mutate({{ condition }} := forcats::fct_relevel({{ condition }}, "combined")) %>%
-        dplyr::select(-.data$type) %>%
+        dplyr::select(-"type") %>%
         dplyr::group_by({{ condition }}) %>%
         dplyr::mutate(median = stats::median(.data$values)) %>%
         dplyr::distinct()
diff --git a/man/assign_peptide_type.Rd b/man/assign_peptide_type.Rd
index 5f0da6ee..baff1587 100644
--- a/man/assign_peptide_type.Rd
+++ b/man/assign_peptide_type.Rd
@@ -8,7 +8,9 @@ assign_peptide_type(
   data,
   aa_before = aa_before,
   last_aa = last_aa,
-  aa_after = aa_after
+  aa_after = aa_after,
+  protein_id = protein_id,
+  start = start
 )
 }
 \arguments{
@@ -23,6 +25,13 @@ acid as one letter code.}
 
 \item{aa_after}{a character column in the \code{data} data frame that contains the following amino
 acid as one letter code.}
+
+\item{protein_id}{a character column in the \code{data} data frame that contains the protein
+accession numbers.}
+
+\item{start}{a numeric column in the \code{data} data frame that contains the start position of
+each peptide within the corresponding protein. This is used to check if the protein is consistently
+missing the initial Methionine, making peptides starting at position 2 "tryptic" on that site.}
 }
 \value{
 A data frame that contains the input data and an additional column with the peptide
@@ -34,14 +43,18 @@ Peptides with preceeding and C-terminal lysine or arginine are considered fully-
 peptide is located at the N- or C-terminus of a protein and fulfills the criterium to be
 fully-tryptic otherwise, it is also considered as fully-tryptic. Peptides that only fulfill the
 criterium on one terminus are semi-tryptic peptides. Lastly, peptides that are not fulfilling
-the criteria for both termini are non-tryptic peptides.
+the criteria for both termini are non-tryptic peptides. In addition, peptides that miss the initial
+Methionine of a protein are considered "tryptic" at that site if there is no other peptide
+starting at position 1 for that protein.
 }
 \examples{
 data <- data.frame(
-  aa_before = c("K", "S", "T"),
-  last_aa = c("R", "K", "Y"),
-  aa_after = c("T", "R", "T")
+  aa_before = c("K", "M", "", "M", "S", "M", "-"),
+  last_aa = c("R", "K", "R", "R", "Y", "K", "K"),
+  aa_after = c("T", "R", "T", "R", "T", "R", "T"),
+  protein_id = c("P1", "P1", "P3", "P3", "P2", "P2", "P2"),
+  start = c(38, 2, 1, 2, 10, 2, 1)
 )
 
-assign_peptide_type(data, aa_before, last_aa, aa_after)
+assign_peptide_type(data, aa_before, last_aa, aa_after, protein_id, start)
 }
diff --git a/tests/testthat/test-auxiliary_functions.R b/tests/testthat/test-auxiliary_functions.R
index a3012779..c08c0aa9 100644
--- a/tests/testthat/test-auxiliary_functions.R
+++ b/tests/testthat/test-auxiliary_functions.R
@@ -11,14 +11,18 @@ if (Sys.getenv("TEST_PROTTI") == "true") {
   })
 
   protein <- fetch_uniprot(uniprot_ids = "P36578")
+  protein2 <- fetch_uniprot(uniprot_ids = "P00925")
   if (!is.null(protein)) {
     data <- tibble::tibble(
-      protein_id = rep("P36578", 3),
-      protein_sequence = rep(protein$sequence, 3),
+      protein_id = c(rep("P36578", 3), rep("P00925", 3)),
+      protein_sequence = c(rep(protein$sequence, 3), rep(protein2$sequence, 3)),
       peptide = c(
         stringr::str_sub(protein$sequence, start = 87, end = 97),
         stringr::str_sub(protein$sequence, start = 59, end = 71),
-        stringr::str_sub(protein$sequence, start = 10, end = 18)
+        stringr::str_sub(protein$sequence, start = 10, end = 18),
+        stringr::str_sub(protein2$sequence, start = 5, end = 10),
+        stringr::str_sub(protein2$sequence, start = 2, end = 15),
+        stringr::str_sub(protein2$sequence, start = 10, end = 15)
       )
     )
 
@@ -29,12 +33,18 @@ if (Sys.getenv("TEST_PROTTI") == "true") {
             protein_sequence = protein_sequence,
             peptide_sequence = peptide
           ) %>%
-          peptide_type(aa_before = aa_before, last_aa = last_aa))
+          peptide_type(
+            aa_before = aa_before,
+            last_aa = last_aa,
+            aa_after = aa_after,
+            protein_id = protein_id,
+            start = start
+          ))
       })
       expect_is(assigned_types, "data.frame")
-      expect_equal(nrow(assigned_types), 3)
+      expect_equal(nrow(assigned_types), 6)
       expect_equal(ncol(assigned_types), 9)
-      expect_equal(assigned_types$pep_type, c("fully-tryptic", "semi-tryptic", "non-tryptic"))
+      expect_equal(assigned_types$pep_type, c("fully-tryptic", "semi-tryptic", "non-tryptic", "non-tryptic", "fully-tryptic", "fully-tryptic"))
     })
 
     assigned_types <- data %>%
@@ -42,13 +52,18 @@ if (Sys.getenv("TEST_PROTTI") == "true") {
         protein_sequence = protein_sequence,
         peptide_sequence = peptide
       ) %>%
-      assign_peptide_type(aa_before = aa_before, last_aa = last_aa)
+      assign_peptide_type(
+        aa_before = aa_before,
+        last_aa = last_aa,
+        aa_after = aa_after,
+        protein_id = protein_id
+      )
 
     test_that("find_peptide and assign_peptide_type work", {
       expect_is(assigned_types, "data.frame")
-      expect_equal(nrow(assigned_types), 3)
+      expect_equal(nrow(assigned_types), 6)
       expect_equal(ncol(assigned_types), 9)
-      expect_equal(assigned_types$pep_type, c("fully-tryptic", "semi-tryptic", "non-tryptic"))
+      expect_equal(assigned_types$pep_type, c("fully-tryptic", "semi-tryptic", "non-tryptic", "non-tryptic", "fully-tryptic", "fully-tryptic"))
     })
 
     test_that("deprecated sequence_coverage works", {
@@ -60,9 +75,9 @@ if (Sys.getenv("TEST_PROTTI") == "true") {
         ))
       })
       expect_is(coverage, "data.frame")
-      expect_equal(nrow(coverage), 3)
+      expect_equal(nrow(coverage), 6)
       expect_equal(ncol(coverage), 10)
-      expect_equal(unique(round(coverage$coverage, digits = 1)), 7.7)
+      expect_equal(unique(round(coverage$coverage, digits = 1)), c(7.7, 3.2))
     })
 
     coverage <- calculate_sequence_coverage(
@@ -73,14 +88,14 @@ if (Sys.getenv("TEST_PROTTI") == "true") {
 
     test_that("calculate_sequence_coverage works", {
       expect_is(coverage, "data.frame")
-      expect_equal(nrow(coverage), 3)
+      expect_equal(nrow(coverage), 6)
       expect_equal(ncol(coverage), 10)
-      expect_equal(unique(round(coverage$coverage, digits = 1)), 7.7)
+      expect_equal(unique(round(coverage$coverage, digits = 1)), c(7.7, 3.2))
     })
 
     plot_data <- coverage %>%
       dplyr::mutate(
-        fold_change = c(3, -0.4, 2.1),
+        fold_change = c(3, -0.4, 2.1, 0.1, -0.1, 0.2),
         protein_length = nchar(protein_sequence)
       )
 
@@ -106,7 +121,7 @@ if (Sys.getenv("TEST_PROTTI") == "true") {
         end_position = end,
         protein_length = protein_length,
         coverage = coverage,
-        protein_id = protein_id,
+        facet = protein_id,
         colouring = pep_type
       )
       expect_is(p, "ggplot")
diff --git a/vignettes/data_analysis_single_dose_treatment_workflow.Rmd b/vignettes/data_analysis_single_dose_treatment_workflow.Rmd
index 8cb8a6d4..95c0d397 100644
--- a/vignettes/data_analysis_single_dose_treatment_workflow.Rmd
+++ b/vignettes/data_analysis_single_dose_treatment_workflow.Rmd
@@ -202,7 +202,8 @@ data_filtered_uniprot <- data_filtered_proteotypic %>%
   assign_peptide_type(
     aa_before = aa_before,
     last_aa = last_aa,
-    aa_after = aa_after
+    aa_after = aa_after,
+    protein_id = pg_protein_accessions
   ) %>%
   calculate_sequence_coverage(
     protein_sequence = sequence,