A 13 year old male patient complains about the discoloration of his teeth. His mother shows a similar discoloration of her teeth. Clinical findings reveal teeth with mottled, opaque white brown yellowish discoloration. Snowcapped appearance of teeth is appreciable. OPG reveals a thin layer of enamel over the cusp tips and interproximal surfaces. Probe test is positive. Based on the findings, the defective gene is: