CAST and Hearing impairment: Of these recombinant mice, 30 had flanking markers with B6/CAST and B6/B6 genotypes and were evaluated for hearing impairment by ABR; 18 of these had significantly elevated ABR thresholds (>20 dB above normal) when tested at 4–6 weeks of age and were classified as rda/rda mutants; the other 12 mice had normal thresholds and were classified as +/rda.