The process of human embryo implantation is mediated not only by evolutionarily conserved mechanisms but by activities unique to humans. Among the latter, evidence suggests that the cell adhesion molecule trophinin plays a unique role in human embryo implantation. Here, we describe characteristics of trophinin protein and of the trophinin-associated proteins bystin and tastin. We then describe trophinin-mediated signal transduction in trophectoderm cells during human embryo implantation and events related to human sperm tail motility. We also report dual roles for trophinin in human cancers in terms of promoting malignancy in some tumor types and suppressing it in others.