One hypothesis suggests that certain human leukocyte antigen (HLA) genes in the MHC locus on chromosome 6 may favor the development of SIgAD and autoimmune diseases, or that an unknown genetic defect could explain the association between SIgAD and autoimmunity, characterizing this phenotype of SIgAD as a monogenic disease.22 The gene discussed is HLA-C; the disease is selective IgA deficiency disease.