TGFBI and Thiel-Behnke corneal dystrophy: Of the mutations at position 555 in exon 12, the R555W mutation causes GCD1 and the R555Q mutation causes Thiel-Behnke corneal dystrophy.3,11 Different mutations in the TGFBI gene, even if they occur at the same position, have been shown to cause different corneal dystrophies by altering folding, proteolysis, accumulation, and half-life (turnover) of the protein.15,16,17,18