HLA-B27, or Human Leukocyte Antigen B27, is a specific genetic marker or allele that is part of the major histocompatibility complex (MHC) class I gene group. HLA genes encode proteins on the surface of cells that play a crucial role in the immune system, specifically in distinguishing self from non-self antigens. HLA-B27 is known for its association with certain autoimmune diseases, particularly ankylosing spondylitis and other spondyloarthropathies.