The ectodomain of LIN-12/Notch proteins is cleaved and shed upon ligand binding. In Caenorhabditis elegans, genetic evidence has implicated SUP-17, the ortholog of Drosophila Kuzbanian and mammalian ADAM10, as the protease that mediates this event. In mammals, however, biochemical evidence has implicated TACE, a different ADAM protein. We have investigated potential functional redundancy of sup-17 and the C. elegans ortholog of TACE, adm-4, by exploring their roles in cell fate decisions mediated by lin-12/Notch genes. We found that reduced adm-4 activity, like reduced sup-17 activity, suppresses an allele of glp-1 that encodes a constitutively active receptor. Furthermore, concomitant reduction of adm-4 and sup-17 activity causes the production of two anchor cells in the hermaphrodite gonad, instead of one--a phenotype associated with loss of lin-12 activity. Concomitant reduction of both sup-17 and adm-4 activity in hermaphrodites results in highly penetrant synthetic sterility, which appears to reflect a defect in the spermatheca. Expression of a truncated form of LIN-12 that mimics the product of ectodomain shedding rescues this fertility defect, suggesting that sup-17 and adm-4 may mediate ectodomain shedding of LIN-12 and/or GLP-1. Our results are consistent with the possibility that sup-17 and adm-4 are functionally redundant for at least a subset of LIN-12/Notch-mediated decisions in C. elegans.