Exon 1 was replaced by a construct containing lacZ and neo. The deleted coding sequence encodes the entire N-terminal dimerization domain and a portion of another domain showing homology to either the human POU-A subdomain or human B-domain. Western blot analysis showed an absence of encoded protein in homozygous mutant embryos. Beta-galactosidase was expressed under the control of the endogenous promoter. (J:58079)