Exons 2 through 5 were replaced with a N-terminally Myc-tagged cDNA containing nucleotide substitutions that result in the amino acid substitution of arginine for lysine at positions 285 and 399 (K285R, K399R). Flp-mediated recombination removed an FRT- and loxP-flanked neomycin resistance cassette inserted downstream of the inserted cDNA. The protein product is nonubiquitinatable and phenotypic analysis indicates a null allele. (J:202106)