Sequence encoding S80, a phosphorylation mutant allele of the orthologous rat gene in which 7 serine codons are replaced with alanine codons, was inserted in place of a fragment extending from the start codon (bp 7957) to the end of the homeobox (bp 9480). A polyadenylation signal and neo cassette were included immediately downstream of S80. Southern blot analysis confirmed the integration of the mutation that putatively produces a protein lacking residues required for phosphorylation.