Syndrome, Martin-Bell: Better known as the fragile X syndrome, the most common heritable form of mental retardation. Fragile X syndrome is due to mutation (changes) at the fragile X site and so perforce is X-linked (carried on the X chromosome). Although it is usually more severe in males than females, the syndrome is due to a dynamic mutation (a trinucleotide repeat) that can change in length and hence in severity from generation to generation, from person to person, and even within a given person. The fragile X syndrome is known as the Martin-Bell syndrome in honor of their discovery of it in 1943.