Histocompatibility in HSCT is more than just identifying a 10/10 HLA-matched donor. Have you ever wondered about how HLA-DPB1 (DP) is different to other HLA loci and why we use different terms like “permissive” and “nonpermissive mismatches”? Ever wanted a (very) basic introduction to killer cell immunoglobulin-like receptor (KIR) and how some places use KIR typing in donor selection? And how about the different types of donor chimerism and their clinical significance when it is not a straightforward 100% chimerism result? In this session, we will bring a case-based approach in highlighting key teaching points on some of these aspects of histocompatibility. Don’t worry if you do not have prior knowledge; we will provide the introductions. This session is categorized as “Intermediate” only because we going beyond the basics of HLA. This session is an extension of and inspired by feedback from last year’s session “A Step Above the Basics of HLA for Apheresis/Transfusion Practitioners”.