sentences:  The arrangement of these heterodimers determines the 3D structure of the knobbed spike on the viral surface.The Env proteins of the Avian Sarcoma and Leukosis virus (ASLV) and the Murine Leukemia Virus (MLV) are both trimers of SU-TM heterodimers.The Env protein of Human Immunodeficiency Virus (HIV) also has a trimeric structure of heterodimers.