A vector was constructed that contained an in-frame fusion of 1.5 kb 5' genomic sequences of the targeted gene (up to exon 8) to the nuclear localization signal (nls) of the SV40 T antigen and the E. coli lacZ gene. After homologous recombination in ES cells, only exons 1 - 7 of the targeted gene were intact and were fused to the lacZ reporter. Western blots of total cell lysates from spleen cells and from thymocytes showed no detectable protein for the targeted gene in hemizygous mutant male mice.