Fig. 10. Two stages in ref-2-dependent patterning of Pn.p cell fusion. (A) A model for how REF-2 prevents Pn.p cell fusion. REF-2 and a Hox protein (LIN-39 or MAB-5) act by cooperatively inhibiting a common set of fusion gene targets. Alternatively, the two transcription factors could together activate a gene(s) required to prevent cell fusion. (B) A model for how ref-2 expression might be regulated. In Pn.p cells in males, either Hox protein alone (LIN-39 or MAB-5) can activate ref-2 transcription and thus can keep the concentration of REF-2 high in the Pn.p cells when the cell fusion decision is made. However, when both Hox proteins are present, ref-2 expression decreases in the Pn.p cells and those cells fuse with hyp7. The mu218 mutation allows ref-2 to be expressed even in cells containing both LIN-39 and MAB-5 (mu218 could affect the ability of Hox proteins to bind ref-2 regulatory sites).

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