Nuclear transcripts of mouse heavy chain immunoglobulin genes contain only the expressed class of C-region sequences.

In plasmacytoma cells producing IgG, IgA, or IgM immunoglobulin heavy chains, the large precursors of the heavy chain messenger RNA's contain nucleotide sequences that specify only the expressed class of constant region. This indicates that the switch from one class of heavy chain to another during B cell ontogeny does not occur by altered processing of a complex gene transcript.