Ordering of mouse immunoglobulin heavy chain genes by molecular cloning

We have determined the order of the mouse immunoglobulin gamma 1, gamma 2 b, gamma 2 a and epsilon genes by molecular cloning of overlapping chromosomal segments. The results clearly demonstrate that the order is 5'-gamma 1-(21 kilobases)-gamma 2b-(15 kilobases)-gamma 2a-(15 kilobases)-epsilon-3'. There seem to be no J regions at the 5' side of each constant region gene so far obtained except for the mu gene and these constant region genes seem to have repetitive sequences characteristic of switch (S) regions at their 5' side.