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Four point mutations were introduced: A G-to-T transverion at nucleotide 1546, prediced to alter amino acid 349 from glycine to cysteine; A C-to-T transition at nucleotide 1551 to create a GUC ribozyme cleaveage site and C-to-A and C-to-G mutations in codon 356 to alter the corresponding leucine to a methionine. A loxP-flanked neomycin selection cassette containing stop codons in all three reading frames was also inserted into intron 22. RT-PCR analysis on RNA from E18.5 homozygous embryos demonstrated that aberrant transcripts are produced from this allele that incorporate sequences from the stop-neomycin cassette, resulting in the expression of a truncated protein, shown by western blot on cultured fibroblasts derived from homozygous mice. (J:59168)