A new blood test identifies the X chromosome mutation underlying fragile X syndrome. (Christine Harrison / Science Photo Library)

A new blood test can identify within seven hours whether a person carries the genetic mutation underlying fragile X syndrome, one of the leading genetic markers of autism. Previously, it could take as long as two weeks to receive the results of a blood test for fragile X syndrome. The new test is intended to be used in conjunction with family history and behavioral observation and cannot be used on fetuses.