diff --git a/src/alphabets/protein.rs b/src/alphabets/protein.rs
index 5bb72c8960..dd7311b389 100644
--- a/src/alphabets/protein.rs
+++ b/src/alphabets/protein.rs
@@ -21,6 +21,11 @@ pub fn alphabet() -> Alphabet {
     Alphabet::new(&b"ARNDCEQGHILKMFPSTWYVarndceqghilkmfpstwyv"[..])
 }
 
+/// Returns the IUPAC amino acid alphabet.
+pub fn iupac_alphabet() -> Alphabet {
+    Alphabet::new(b"ABCDEFGHIKLMNPQRSTVWXYZabcdefghiklmnpqrstvwxyz")
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -44,4 +49,9 @@ mod tests {
     fn number_is_no_word() {
         assert!(!alphabet().is_word(b"42"));
     }
+
+    #[test]
+    fn iupac_contains_iupac_chars() {
+        assert!(iupac_alphabet().is_word(b"XMN"));
+    }
 }