The most popular blood group antigen system, ABO antigens had been known to consist of alpha1,2-fucosylated galactose substituted with alpha1,3-N-acetylgalactosamine (A) or galactose (B). These biochemical results were clearly confirmed and further investigated by the molecular cloning of blood group A synthase cDNA (Yamamoto et al. 1990a, 1990b). It was demonstrated that blood group A synthase gene is located at the human 9th chromosome ABO locus with blood group B synthase as an allelic gene having 7 nucleotides mutation in the coding region (Fig. 1). Blood group O gene is also present at the same locus with no enzymatic activity due to one nucleotide deletion. As for P1/P/pk/p blood group system, P/pk/p blood group system has been clarified to be based on globo-series glycosphingolipids, i.e., P is globoside (Gb4, globotetraosylceramide) (Okajima et al. 2000), pk is Gb3 (globotriaosylceramide) that is accumulated in the deficiency of Gb4 synthase, and p means individuals lacking the activity of Gb3 synthesis (Furukawa et al. 2000; Table 1). P1 is, in turn, alpha4Gal- structure substituted on a neolacto-series core structure. There has been a long-time controversy on the identity between P1 synthase and Gb3 synthase (Kojima et al. 2000; Table 2). Iwamura et al. (2003) demonstrated P1 is synthesized by Gb3 synthase, suggesting that there should be differences in the transcription efficiency of Gb3 synthase gene between P1 and P2 individuals.