Expression of membrane (mIg) and secreted (sIg) forms of Igμ during development, estimated by RT-PCR. Total RNA was prepared from 50–100 fish at each time point. PCR was performed with an upstream primer, Cμ3S, corresponding to a segment of the CH3 domain, which is found in both the secreted and membrane forms of Igμ; the downstream primers, Ctail and tm2, are specific for the secreted and membrane carboxyl-terminal tail pieces, respectively. The expression of elongation factor 1α served as a standard.