u heavy chain that lacks the cysteine in the tailpiece. In summary, hexameric IgM never contains the J chain; pentameric IgM can be formed so as to include or not include the J chain.

An important difference between the u and ? heavy chains is the availability of cysteines for forming disulfide bonds between heavy chains. In the case of the ? heavy chain, the only inter-? bonds are formed by cysteines in the hinge, and accordingly, each ? chain binds to only one other ? chain. By contrast, the Cu2 and Cu3 domains and the tailpiece each