The butyrophilin family is divided into two subfamilies: the classical butyrophilins (BTN) and the butyrophilin-like (BTNL) proteins. The classical butyrophilins are further divided into three groups: BTN1A1, BTN2A1, and BTN3A1. The BTNL proteins are structurally similar to the classical butyrophilins but lack the conserved intracellular domain.
