The P blood group antigen of the GLOB system is a glycolipid structure, also known as globoside, on the red blood cells (RBCs) of almost all individuals worldwide. The P antigen is intimately related to the Pkand NOR antigens discussed in the review about the P1PK blood group system. Naturally occurring anti-P is present in the serum of individuals with the rare globoside-deficient phenotypes p, P1k, and P2kand has been implicated in hemolytic transfusion reactions as well as unfavorable outcomes of pregnancy. The molecular genetic basis of globoside deficiency is absence of functional P synthase as a result of mutations at the