Phemx is a member of the tetraspanin (TM4SF) family of proteins that may be involved in transmembrane signal transduction, regulation of cell proliferation, differentiation and motility. Phemx is a multi-pass membrane protein containing intracellular N- and C-terminal domains, four transmembrane domains and two extracellular loops. It is ubiquitously expressed from early embryogenesis through adulthood. Phemx exhibits predominant expression in hematopoietic tissues suggesting a role in hematopoietic-cell function. In association with the Integrin αIIb/Integrin β3 complex, Phemx functions to stabilize arterial thrombi in platelets and regulate “outside-in” signaling. This interaction may be important in the process of wound healing. The gene encoding Phemx is located in an important tumor-suppressor gene region that has been associated with Beckwith-Wiedemann syndrome as well as a variety of cancers.