diff --git a/python/obitools3/dms/obiseq.pyx b/python/obitools3/dms/obiseq.pyx
index beed690..655f100 100644
--- a/python/obitools3/dms/obiseq.pyx
+++ b/python/obitools3/dms/obiseq.pyx
@@ -26,6 +26,7 @@ SEQUENCE_COLUMN = b"SEQ"
 
 SPECIAL_COLUMNS = [NUC_SEQUENCE_COLUMN, SEQUENCE_COLUMN, ID_COLUMN, DEFINITION_COLUMN, QUALITY_COLUMN]
 
+QUALITY_ASCII_BASE = 33
 
 
 cdef class Seq(dict) :
@@ -127,7 +128,7 @@ cdef class Seq(dict) :
         
 cdef class Nuc_Seq(Seq) :
 
-    def __init__(self, object id, object seq, object definition=None, object quality=None, int offset=-1, object tags=None) :
+    def __init__(self, object id, object seq, object definition=None, object quality=None, int offset=QUALITY_ASCII_BASE, object tags=None) :
         cdef object k 
         cdef bytes  k_b
         cdef int    q
@@ -183,7 +184,7 @@ cdef class Nuc_Seq(Seq) :
         else:
             return None
     
-    cpdef set_quality(self, object new_quality, int offset=-1):
+    cpdef set_quality(self, object new_quality, int offset=QUALITY_ASCII_BASE):
         cdef list quality_int
         if type(new_quality) == list:
             quality_int = new_quality
@@ -290,7 +291,7 @@ cdef class Nuc_Seq_Stored(Seq_Stored) :
 #             if column_name_b in SPECIAL_COLUMNS:
 #                 self[column_name_b] = tags[column_name]
 
-    cpdef set(self, object id, object seq, object definition=None, object quality=None, int offset=-1, object tags=None):
+    cpdef set(self, object id, object seq, object definition=None, object quality=None, int offset=QUALITY_ASCII_BASE, object tags=None):
         cdef object k 
         cdef bytes  k_b
         cdef int    q
@@ -336,7 +337,7 @@ cdef class Nuc_Seq_Stored(Seq_Stored) :
             Column.new_column(self._view, QUALITY_COLUMN, OBI_QUAL)
         self._view.get_column(QUALITY_COLUMN).set_line(self.index, new_qual)
 
-    cpdef set_quality_char(self, object new_qual, int offset=-1):
+    cpdef set_quality_char(self, object new_qual, int offset=QUALITY_ASCII_BASE):
         if QUALITY_COLUMN not in self:
             Column.new_column(self._view, QUALITY_COLUMN, OBI_QUAL)
         self._view.get_column(QUALITY_COLUMN).set_str_line(self.index, tobytes(new_qual), offset=offset)