The underlying mutation responsible for the phenotype in the cocoa mouse was identified as an A-to-T substitution in the intron 14-exon 15 splice acceptor site, changing it from TAG to TTG. With the endogenous acceptor eleminated, a cryptic splice site is utilized within exon 15, which results in a frameshift and premature termination. Northern analysis demonstrated qualitative and quantitative differences in mRNA between wild-type and homozygous mutant animals. (J:72606)