The IgM molecule is composed of subunits made up of two light chain and two heavy chain (mu) polypeptides. The mu chain is encoded by several gene segments--variable (V), joining (J) and constant (Cmu). The Cmu gene segment is of particular interest for several reasons. First, the mu chain must exist in two very different environments--as an integral membrane protein in receptor IgM molecules (micrometer) and as soluble serum protein in IgM molecules into the blood (mus). Second, the Cmu region in mus is composed of four homology units or domains (Cmu1, Cmu2, Cmu3 and Cmu4) of approximately 110 amino acid residues plus a C-terminal tail of 19 residues. We asked two questions concerning the organisation of the Cmu gene segment. (1) Are the homology units separated by intervening DNA sequences as has been reported for alpha (ref. 5), gamma 1 (ref. 6) and gamma 2b (ref. 7) heavy chain genes? (2) Is the C-terminal tail separated from the Cmu4 domain by an intervening DNA sequence? If so, DNA rearrangements or RNA splicing could generate hydrophilic and hydrophobic C-terminal tails for the mus and micrometer polypeptides, respectively. We demonstrate here that intervening DNA sequences separate each of the four coding regions for Cmu domains, and that the coding regions for the Cmu4 domains and the C-terminal tail are directly contiguous.