A portion of exon 4 containing the helix-loop-helix coding domain and all of exon 5 were replaced by a neomycin selection cassette. A larger transcript, resulting from a cryptic splice site between exon 3 and the neomycin resistance gene, was detected by Northern blot analysis. The polypeptide truncates 15 residues after this splice site, prior to the functional domains encoded by exon 4 in the wild-type locus.