The high affinity receptor for immunoglobulin G CD64 is encoded by a family of 3 genes that share over 98% of DNA sequence homology: FCGR1A,FCGR1B and FCGR1C. The FCGR1C gene,like FCGR1A,is located on 1q21. The third gene of this family,FCGR1B,was found at 1p12. CD64 plays a pivotal role in the immune response. It is a glycoprotein (MW 72 kD) that is constitutively expressed on human monocytes and macrophages.