- Cysteinuria (cysteine in the urine) is a disease involving defective cysteine carrier proteins in the kidney cell membranes.  This transport system normally removes cysteine from the fluid destined to become urine and returns this essential amino acid to the blood.  When this carrier malfunctions, large quantities of cysteine remain in the urine, where it is relatively insoluble and tends to precipitate.  This is one cause of urinary stones.