The insertion of a PGK-neo cassette disrupted the exon encoding the phosphotyrosine binding (PTB) domain. Western blot analysis confirmed an absence of normal protein product in homozygous mutant mice. The presence of a truncated protein was noted and determined to be the result of an in-frame splicing event in which the targeted exon was deleted, thus all peptides produced from targeted alleles lacked the conserved residues within the PTB domain.