DEFB1 and chronic obstructive pulmonary disease: DEFA2 is the proteolytic product of DEFA1 and/or DEFA3; DEFA1 and DEFA3 are encoded by separate genes.[16] Due to a copy number polymorphism, individuals can inherit variable copy numbers of the DEFA1 and DEFA3 genes, and the DEFA3 gene is often absent.[16] Genetic variation in the DEFB1 gene is associated with COPD phenotypes in Japanese [17] and Chinese [18] populations.