RRM2B and focal segmental glomerulosclerosis: In children renal involvement present most commonly in the form of proximal tubulopathy (mutations in RRM2B) or as steroid-resistant nephrotic syndrome while in adulthood focal segmental glomerulosclerosis is more common. The tubulopathy may be of the Fanconi type or more similar to Gitelman's syndrome with severe hypomagnesemia.