The sel-6 gene was previously identified in a screen for suppressors of the egg-laying defect associated with hypermorphic alleles of lin-12 (Tax et al. 1997). Here we show that sel-6 and two other previously defined genes, mal-2 and emb-4, are the same gene, now called "emb-4." We perform a genetic and molecular characterization of emb-4 and show that it functions cell autonomously as a positive regulator of lin-12 activity. Viable alleles identified as suppressors of lin-12 are partial loss-of-function mutations, whereas the null phenotype encompasses a range of lethal terminal phenotypes that apparently are not related to loss of lin-12/Notch signaling. emb-4 encodes a large nuclearly localized protein containing a predicted ATPase domain and has apparent orthologs in fission yeast, plants, and animals.