The RP24-297F14 bacterial artificial chromosome (BAC), from the RPCI-24 mouse genomic library derived from a C57BL/6J male mouse, was modified by replacement of the sequence from the translation initiation site through the 3' untranslated region (3'-UTR) by the orthologous human sequence. A FRT site-flanked neomycin selection cassette was inserted ~1 kb downstream of the humanized 3'-UTR. This modified BAC was then used to target the endogenous mouse gene. (J:169366)