B*15 is the largest allele grouping for any known human autosomal locus, identified as of August 2008 there are more than 150 alleles and ~140 amino acid sequence variants from those gene products.
As people traveled east the frequency of many alleles dropped or disappeared from migrants.
[2] The wide range and complex environment selected for new alleles and promoted their expansion.
B*46 for example is not found in Africa, and appears to have evolved and spread in East Asia, to several 100 million bearers worldwide.
HLA-B15 allele *15:02 is associated with the severe skin conditions Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) caused by carbamazepine drug sensitivity in East Asians.