An approximately 4 kb fragment, including exons 7 - 9 encoding for the POU homeodomain and a part of the POU?specific domain, was replaced with a tk-neo cassette via homologous recombination. In situ and immunohistochemical analysis indicated the expression of a truncated mRNA in a small population of basal cells and the absence of the a isoform of the protein in the taste buds of homozygous mice. (J:173905)