The aim of this study was three-fold: (1) to delineate the clinical spectrum of mono-allelic variants in COL4A3 or COL4A4; (2) to identify genotype-phenotype correlations for the identified variants; and (3) to characterize kidney histology of patients with a mono-allelic variant in COL4A3 and COL4A4 and compare this with other causes of FSGS and other inherited kidney diseases. The gene discussed is COL4A3; the disease is focal segmental glomerulosclerosis.