Background/Aims: A subset of patients who present with proteinuria and are diagnosed with focal segmental glomer- ulosclerosis (FSGS) have inherited heterozygous COL4A3/A4 mutations and are also diagnosed with thin basement mem- brane nephropathy (TBMN-OMIM: 141200). Two studies showed that co-inheritance of NPHS2-p.Arg229Gln, a podo- cin variant, may increase the risk for proteinuria and renal function decline. Methods: We hypothesized that additional podocin variants may exert a similar effect. We studied ge- netically a well-characterized Cypriot TBMN patient cohort by re-sequencing the NPHS2 coding region. We also per- formed functional studies in cell culture experiments, inves- tigating the interaction of podocin variants with itself and with nephrin. Results: Potentially disease-modifying podocin variants were searched for by analyzing NPHS2 in 35 ‘severe’ TBMN patients. One non-synonymous variant, p.Glu237Gln, was detected. Both variants, p.Arg229Gln and p.Glu237Gln, were tested in a larger cohort of 122 TBMN patients, who were categorized as ‘mild’ or ‘severe’ based on the presence of microscopic hematuria alone or com- bined with chronic renal failure and/or proteinuria. Seven ‘severe’ patients carried either of the 2 variants; none was present in the ‘mild’ patients (p = 0.05, Pearson χ2). The 7 carriers belong in 2 families segregating mutation COL4A3-p.Gly1334Glu. Inheritance of the wild-type (WT) and mutant alleles correlated with the phenotype (combined concordance probability 0.003). Immunofluorescence (IF) experiments after dual co-transfection of WT and mutant podocin suggested altered co-localization of mutant ho- modimers. IF experiments after co-transfection of WT podo- cin and nephrin showed normal membrane localization, while both podocin variants interfered with normal traffick- ing, demonstrating perinuclear staining. Immunoprecipita- tion experiments showed stronger binding of mutant podo- cin to WT podocin or nephrin. Conclusion: The results sup- port the hypothesis that certain hypomorphic podocin variants may act as adverse genetic modifiers when co-in- herited with COL4A3/A4 mutations, thus predisposing to FSGS and severe kidney function decline.