Butyrophilin Subfamily 3 Member A1 (BTN3A1/CD277) is a type I transmembrane glycoprotein member of the Ig superfamily. It is expressed on a wide variety of immune cells. Similar to BTN3A2 and BTN3A3, BTN3A1 is composed of an extracellular N-terminal IgV and a membraneproximal IgC domain followed by a transmembrane domain and a cytoplasmic tail. These Ig domains are also found in B7 family costimulatory molecules, suggesting structural and functional similarities between the two protein families. BTN3A1 acts as a critical protein for the activation of Vγ9Vδ2 T cells following detection of distressed cells. The anti-tumor responses of Vγ9Vδ2 T cells may be enhanced with agonistic anti-BTNA3 antibodies.