To disrupt phosphorylation of the encoded protein at threonine 87, a threonine-to-asparagine substitution mutation was engineered in the basic DNA-binding region at codon 87 (T87N) of exon 1. A single frt site remained in intron after the excision of an frt-flanked neo cassette from the germline. Expression of the mutant transcript was verified by Northern blot analysis of homozygous mutant mice.