Technical Abstract: GEX1 is a plasma membrane protein conserved among plant species, and was previously shown to be expressed in sperm cells and some sporophytic tissues. Here we show that GEX1 is also expressed in the embryo sac before cellularization, in the egg cell after cellularization, in the zygote/embryo immediately after fertilization, and in the pollen vegetative cell. We functionally characterize GEX1 in Arabidopsis thaliana, and show that it is a versatile protein with functions during male and female gametophyte development and during early embryogenesis. gex1-1/+ plants, which synthesize a truncated GEX1 mRNA encoding a protein lacking the predicted cytoplasmic domain but still targeted to the plasma membrane, had embryos that arrested before the preglobular stage. gex1-3/+ plants, carrying a null GEX1 allele, had defects during male and female gametophyte development and during early embryogenesis. Using an antisense GEX1 transgenic line we demonstrate that the predicted GEX1 extracellular domain is sufficient and necessary for GEX1 function during development of both gametophytes. The predicted cytoplasmic domain is necessary for correct early embryogenesis and mediates homodimer formation at the plasma membrane. We propose that dimerization of GEX1 in the zygote might be an upstream step in a signaling cascade regulating early embryogenesis.