The majority of exon 12, which encodes the kinase subdomains 2 through 4 common to the 3 known isoforms (alpha, beta, and gamma), was replaced by a neomycin selection cassette inserted by homologous recombination. While, RT-PCR analysis showed the presence of an aberrant splice product, sequence analysis showed it generated a frameshift mutation in the open reading frame. Western blot analysis verified the absence of protein.