The pre-B cell receptor (pre-BCR) is composed of the immunoglobulin (Ig) heavy (microH) chain and the surrogate light chain encoded by VpreB and lambda5. The pre-BCR has been implicated in precursor B cell proliferation, differentiation and IgH chain allelic exclusion. B cell development in mice lacking the transmembrane form of microH chain is blocked at the precursor B cell stage: the cells cannot proliferate or differentiate further and the IgH locus is allelically included. In mice lacking lambda5, the precursor B cells, although unable to proliferate, can nonetheless differentiate, whereas the IgH locus is allelically excluded. It was, therefore, postulated that microH chain together with VpreB could form a pre-BCR-like receptor that would allow IgH allelic exclusion but not proliferation. In mice lacking both VpreB genes, precursor B cells do not proliferate but are able to differentiate. Surprisingly, the IgH locus is allelically excluded. This suggests that microH chains find other partner proteins to signal allelic exclusion.