the exon encoding amino acids 23 through 69 was replaced with a construct containing a splice acceptor and a modified partial cDNA encoding the p80 splice form. The cDNA was modified to encode phenylalanines in place of 2 tyrosines residues (220, 232) that are phosphorylation sites. A floxed neomycin selection cassette was excised by transient cre expression. (J:145031)