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 Pk and 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 P2k and 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 B3GALNT1 locus. Other related glycolipid structures, the LKE and PX2 antigens, remain in the GLOB blood group collection pending further evidence about the genes and gene products responsible for their synthesis.