Cell–cell fusogens are proteins that promote plasma membrane fusion among different cells. To be considered a fusogen, it must be required for fusion, fuse unfamiliar membranes, and be present on the fusing membrane when need be. These cells include but are not limited too: gametes, trophoblasts, epithelial, and other developmental cells. These fusogens mediate cell-cell fusion and can perform neuron repairs, auto-fusion, and sealing of the phagosomes. Although these proteins promote similar functions among cells, they have individual mechanisms. These are called unilateral (one fusing membrane needed to be present) and bilateral (same or different fusogens present at both membranes) mechanisms. Most fusogen mechanisms begin with hemifusion, but the mechanism for cell-cell fusogens consists of four separate steps.