Primary IgA nephropathy (IgAN) is a common glomerular disease with a complex genetic architecture. Ethnic differences in susceptibility to IgAN, as well as inter-individual variation in the disease course and prognosis strongly argue for the crucial role of genetic factors in its pathogenesis. For example, IgAN occurs with greatest frequency in Chinese and Japanese populations but is relatively rare in individuals of African descent [1]. A high frequency of IgAN has also been reported in biopsies of Zuni and Manitoba Native Americans and Australian aborigines [2–6]. Familial aggregation of IgAN was first reported in the 1970s, and multiple large series of familial cases have provided further evidence for genetic contribution. Two European studies demonstrated that 4–10% of patients with IgAN had a family history of kidney disease [7, 8]. In other studies, urinary abnormalities were detected in over 20% of asymptomatic first degree relatives of IgAN patients [9]. Several extended kindreds with IgAN have also been reported throughout the world, including the United States [10], France [11], Italy [12], Canada [13], Australia [5] and Lebanon [14]. In all reported families, segregation of IgAN is consistent with autosomal dominant transmission with incomplete penetrance, although more complex genetic models are also compatible with the observed pedigrees. The incomplete penetrance is likely explained by the requirement of additional environmental or genetic factors for clinical manifestation of disease.