diff --git a/python/orgasm/utils/dna.pyx b/python/orgasm/utils/dna.pyx
index ba4e4f6..ad7d020 100644
--- a/python/orgasm/utils/dna.pyx
+++ b/python/orgasm/utils/dna.pyx
@@ -49,7 +49,7 @@ cpdef bytes reverseComplement(bytes seq):
 
 cpdef bint isDNA(bytes seq):
     cdef set c = set([x for x in seq.upper()])
-    return len(c)==4
+    return len(c)<=4
 
 
 cdef dict codon = {}