In this cross-sectional population study, selectivity index, which is based on the comparison of the clearance of IgG (molecular weight 170,000 Daltons), as a marker of high molecular weight proteins, and that of transferrin (molecular weight 80,000 Daltons), as a marker of intermediate-sized proteins [30], was low, and there was a significant association of reduced serum FT3 levels and high selectivity of proteinuria (low selectivity index) in patients with nephrotic-range proteinuria. The gene discussed is TF; the disease is Nephrotic range proteinuria.