A mutation was introduced into exon 2 resulting in serine codon 18 (AGC) to change to alanine (GCC) (p.S18A). A loxP site flanked neomycin resistance gene cassette was inserted into intron 1. Subsequent excision of the neo cassette occurred in the F1 progeny of chimeric males as the result of expression of the protamine-Cre transgene which is endogenous to the PC3 ES cells used.