The PT1-ATG gene trap vector has integrated into the endogenous gene 34 base pairs upstream of the ATG translation initiation site. This vector contains the engrailed 2 (En2) splice acceptor sequence joined to a promoterless lacZ (E. coli beta-galactosidase) reporter gene with an ATG translation initiation codon; it also contains the neomycin phosphotransferase gene under control of the phosphoglycerate kinase 1 (PGK1) promoter (PGK1-neo). (J:82809, J:197093)