Increased sister chromatid exchange frequency at Xq27 site in affected fragile X males.

Sister chromatid exchange (SCE) was evaluated in peripheral blood lymphocytes cultured from normal subjects, fragile X carrier females, and fragile X affected males and in general did not differ among all subjects whether cells were grown in thymidine-deficient or complete medium. However, at the Xq27 site, SCE was significantly increased for fragile X affected males, particularly in cells grown in thymidine-deficient medium.