The Sda histo-blood group antigen [GalNAcβ1-4(NeuAcα2-3)Galβ-R] is present in colon, kidney and body fluids among 96-98% of Europeans whilst 90% have Sda-positive erythrocytes. Sda is implicated in various infections and constitutes a potential biomarker for colon cancer. The 2-4% truly Sd(a‒) individuals may produce anti-Sda, which can lead to incompatible blood transfusions, especially if donors with the high-expressing Sda phenotype, Sd(a++) or Cad, are involved. It was hypothesized that defects in the B4GALNT2-encoded β4GalNAc-T2 glycosyltransferase underlies the null phenotype. We recently reported the association of B4GALNT2 mutations with the Sd(a‒) phenotype, which formally established the SID blood-group system. In the present study, we provide causal proof and glycoprotein profiling underpinning this correlation. Phenotypically Sd(a‒) HEK293 cells were transfected with different B4GALNT2 constructs and evaluated by immunostaining and LC-MS/MS-based glycoproteomics. The pre¬dominant SIDnull allele with SNP rs7224888:T>C (p.Cys406Arg) abolished Sda synthesis, while this antigen was detectable as N- or O-glycans on multiple glycoproteins following transfection of wildtype B4GALNT2. Surprisingly, two rare missense SNPs, rs148441237:A>G and rs61743617:C>T, found in a Sd(a‒) compound heterozygote gave results similar to wildtype. To elucidate if Sd(a++)/Cad is also due to B4GALNT2 alterations, its coding region and 2 kbp upstream were sequenced in five Cad individuals. No genetic changes were associated with this phenotype but a detailed erythroid Cad glycoprotein profile was obtained, especially for GLPA (O-glycosylation) and, for the first time, B3AT (N-glycosylation). In conclusion, the p.Cys406Arg β4GalNAc-T2 variant causes Sda-deficiency in humans, while the enigmatic Cad phenotype remains unresolved, albeit further characterized.