ABO blood group
The ABO blood group system is the best known blood typing system, determing blood group type as A, B, AB, or O, and thereby of importance for blood donors and recipients. Wikipedia
SNPs in the ABO gene determine ABO blood group type.
23andMe checks these 11 snps
The following are also SNPs in the ABO gene, used only to distinguish rare variants or to predict SNPs from the above list (based on linkage):