In the small intestine of heterozygous mice (Dlb-1b/Dlb-1a), the Dlb-1b allele results in a stainable epithelium. The mutation or loss of the dominant Dlb-1b allele in a stem cell results in a non-staining ribbon of cells on a villus of the small intestine. To determine if dominant mutations resulting in the gain of staining--the induction of a Dlb-1b-like allele--could also be detected, we examined Dlb-1a homozygous mice (SWR) 2 weeks after a single treatment with 250 mg/kg ethylnitrosourea. Mutations to the dominant allele should appear as brown ribbons on unstained villi. Such ribbons were observed in the treated group but not in controls. The mutant frequency was low compared to the frequency of Dlb-1a-like mutations reported at the Dlb-1b allele in heterozygous mice.