The replacement of a 1.4 kb genomic fragment with a neo cassette resulted in the disruption of both the Cd247 and Pou2f1 loci, which are located at the same chromosomal position on opposite strands. The deletion of a 3' portion of Cd247 exon 9 and its splice acceptor site disrupted sequence encoding the eta isoform while leaving the sequence encoding the zeta isoform intact. Neither eta transcript nor protein was detected in homozygous mutant mice while normal expression of zeta chain homodimers was confirmed by 2D SDS-PAGE analysis. The insertion of the neo cassette also replaced the 3' UTR of the Pou2f1 locus, resulting in reduced expression of normal transcript as well as in an induced expression of aberrant transcript.