This allele was generated by breeding mice carrying Tcf1tm1Ylee with transgenic mice expressing cre via the EIIa promoter. Cre mediated excision of the floxed fragment, containing the coding region of exon 1 and the neo transgene inserted into intron 1, resulted in the loss of sequence encoding 108 amino acids as well as a portion of intron 1. Transcript was undetected in homozygous mutant mice via RT-PCR analysis.