To generate an allele lacking phosphorylation sites, 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 the 5 tyrosines residues (185, 198, 200, 220, 232) that are potential phosphorylation sites. Phosphorylation levels were undetectable by phosphopeptide analysis of protein produced by the targeted allele in vitro. (J:79475)