Important mutations in human ABO genes are categorized. Different molecular mechanisms may be responsible for seemingly identical phenotypes. For example, the B3 phenotype may be caused by a missense mutation (A301 allele: D291N), a splicing mutation (B303), or the combination of a missense mutation and a single nucleotide deletion, possibly due to recombination (A302: V277M and 1060delC). The kinds of mutations responsible for the ABO subgroups range from missense mutations (A202, A203, A207, A301, A303, Ax01, Ax07, Ax12, Ax13, Bx02, Bx03, etc.), frame-shift mutations due to nucleotide deletions (O01, O02, A206, Ael03, etc.), frame-shift mutations due to nucleotide insertions (Ael01, Bw20, etc.), splicing mutations (Ael04, B303, etc.), an initiation codon mutation (Aw13), and recombination (A302, Aw07, etc.). Nonsense mutations have yet to be found. A localization mutation has recently been found in 2008.