Homologous recombination was used to replace exons 2 and 3 with a beta geo cassette. Northern and Western blot analyses failed to detect transcript and protein, respectively, in mutants. However, a subsequent study has identified a truncated translation product beginning at an alternative start site in exon 5 by using a C-terminal specific antibody. The truncated protein is detectable in cytoplasmic lipid droplets of the secretory epithelial cells in mammary glands after parturition. (J:105572, J:130099)