Adenosine deaminase polymorphisms at the protein and DNA levels

Adenosine deaminase (ADA, E.C. 3.5.4.4) exhibits a well-known polymorphism at the protein level. We have studied ADA and an STR polymorphism exhibiting variation of a TTTA repeat motif at intron 3 of the ADA gene in random samples from northern Portugal (N = 218) and southwestern Germany (N = 114). The ADA phenotype distribution and population data on the worldwide distribution of ADA favor recurrent mutation as an explanation for the maintenance of the ADA*2 gene product at polymorphic frequencies.