A gene trap containing a Beta-geo reporter gene was inserted into intron 5, resulting in a fusion transcript containing the endogenous amino-terminal head domain and a truncated alpha-helical central rod domain fused to Beta-geo. The protein lacked the endogenous carboxyl-terminal 273 amino acids of Lmb1, thereby eliminating one of two known phosphorylation sites, the nuclear localization signal, and the carboxyl-terminal CAAX motif. Only the 5' sequences were detected in mutant embryos by RT-PCR and Western blot showed no protein. (J:91473)