Sequence analysis of the endogenous loci of C57BL/6 and BALB/c mice revealed 9 nucleotide differences between the two backgrounds. Codon 481 on the C57BL/6 background encodes an in-frame stop signal, putatively precluding the translation of the essential glycosylphosphatidylinositol (GPI) anchor. Protein was undetected by Western blot analysis of COS1 cells transfected with the C57BL/6 variant. The absence of fucntional protein was confirmed by a lack of ADP-ribosyltransferase activity in the COS1 cells.