Germ-line transcription occurs on both the functional and the non-functional alleles of immunoglobulin constant heavy chain genes.

In B cells undergoing class switching, recombination is usually directed to the same switch sequences on both homologous chromosomes and is generally preceded by germ-line transcription initiated from I promoters upstream of immunoglobulin constant heavy chain genes. Whether germ-line transcription occurs on both alleles or is restricted to the productive one has not been directly addressed before. To clearly distinguish between these two possibilities, a system is needed in which one might detect allele-specific germ-line transcripts. We used homologous recombination to set up two in vivo systems in which one gamma3 allele was marked by a loxP sequence whereas the other allele was silenced by inhibiting mu heavy chain production. Here, we provide evidence that germ-line transcription occurs on both the functional and the non-functional alleles.