The genetic flaw in X-chromosome spinal-bulbar muscular atrophy (SBMA) is an expanded section of DNA — called a trinucleotide repeat — in a gene that carries instructions for a protein known as the androgen receptor.
The normal function of the androgen receptor is to help cells process androgens (male hormones). When the androgen receptor has extra DNA, it’s longer than it’s supposed to be and may be sticky. The flawed androgen receptor can’t transport male hormones in the right way, and it may "gum up the works" of motor neurons in other ways that keep them from performing their usual functions.
The gene flaw that causes SBMA is on the X chromosome, leading to an X-linked inheritance pattern.
Females have two X chromosomes, and males have an X and a Y chromosome. Females who have a gene flaw on one X chromosome are usually considered carriers of an X-linked disease (although sometimes they can have a mild form of the disease). Males, in contrast, have no second X to protect them and typically show the full effects of the flaw on their single X chromosome.
Each son of a woman who carries an X-linked disease has a 50 percent chance of inheriting the gene flaw and developing the disease. Each daughter has a 50 percent chance of inheriting the gene flaw and being a carrier herself.
Genetic testing via a blood test is available to help diagnose SBMA  and to detect SBMA carriers. For more information on genetics and genetic testing, see the MDA publication Facts About Genetics and Neuromuscular Diseases .