Recent findings by us and others showed that about 50% of patients with COL4A3/COL4A4 heterozygous mutations, a prevalent cause of familial microscopic hematuria due to TBMN, develop proteinuria with secondary focal segmental glomerulosclerosis (FSGS), after their third decade of life. The gene discussed is COL4A4; the disease is focal segmental glomerulosclerosis.