Mice homozygous for the Numbtm1Ynj allele were mated with transgenic mice which express Cre recombinase under the control of the beta-actin promoter. The presence of Cre resulted in germline deletion of sequences in exons 5 and 6 of the Numb gene that encode the middle portion of the phosphotyrosine binding domain of the protein. C terminus immunoreactivity was not detected in protein extracts from homozygous mutant 10.5 day embryos.