Patients with IgA nephropathy have increased serum levels of undergalactosylated IgA1 (gd-IgA1), which has an established role in the development of the disease. In a new study, Lomax-Browne et al. measured the serum levels of IgA and gd-IgA1 in 148 healthy female twins, including 27 monozygotic and 47 dizygotic pairs. Using the classic twin model, they showed that serum levels of gd-IgA1 are 80% heritable and those of IgA are 46% heritable.