EFF-AFF are the identifiers for type 1 glycoproteins that makeup cell–cell fusogens. They were first identified when EFF-1 mutants were found to "block cell fusion in all epidermal and vulval epithelia" in the roundworm, Caenorhabditis elegans. EFF-AFF is a family of type I membrane glycoproteins that act as cell–cell fusogens, named from Anchor cell fusion failure. Because it was known that EFF-1 mutants successfully fused the anchor cell and (uterine seam) utse syncytium to produce a continuous uterine-vulval tube, where these connections failed, AFF-1 mutants were discovered. AFF-1 was deemed necessary for this process in addition to the fusion of heterologous cells in C. elegans. The transmembrane forms of these proteins, like most viral fusogens, possess an N-terminal signal sequence followed by a long extracellular portion, a predicted transmembrane domain, and a short intracellular tail. " A striking conservation in the position and number of all 16 cysteines in the extracellular portion"  of EFF-AFF proteins from different nematode species suggests that these proteins are folded in a similar 3D structure that is
essential for their fusogenic activity. C. elegans AFF-1 and EFF-1 proteins are essential for developmental cell-to-cell fusion and can merge insect cells. "Thus FFs comprise an ancient
family of cellular fusogens that can promote fusion when expressed on a viral particle."