In the present article, we summarize two aspects of our work on mouse ABP (androgen-binding protein): (i) the sexual selection function producing incipient reinforcement on the European house mouse hybrid zone, and (ii) the mechanism behind the dramatic expansion of the Abp gene region in the mouse genome. Selection unifies these two components, although the ways in which selection has acted differ. At the functional level, strong positive selection has acted on key sites on the surface of one face of the ABP dimer, possibly to influence binding to a receptor. A different kind of selection has apparently driven the recent and rapid expansion of the gene region, probably by increasing the amount of Abp transcript, in one or both of two ways. We have shown previously that groups of Abp genes behave as LCRs (low-copy repeats), duplicating as relatively large blocks of genes by NAHR (non-allelic homologous recombination). The second type of selection involves the close link between the accumulation of L1 elements and the expansion of the Abp gene family by NAHR. It is probably predicated on an initial selection for increased transcription of existing Abp genes and/or an increase in Abp gene number providing more transcriptional sites. Either or both could increase initial transcript production, a quantitative change similar to increasing the volume of a radio transmission. In closing, we also provide a note on Abp gene nomenclature.

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