Genetics X-linked X-linked ALD (X-ALD) is the most common form of ALD. In X-ALD, the defective ABCD1 gene resides on the X chromosome (Xq28). The incidence of X-ALD is at least 1 in 20,000 male births. [7] The ABCD1 (“ATP-binding cassette, subfamily D, member 1″) gene was discovered in 1993 and