Rett's disorder, also called Rett's syndrome, is a brain disorder characterized by normal early development (until sometime between 5 months and 4 years) that is followed by a loss of physical and mental growth. Rett's is caused by gene mutation and usually only affects girls; most cases are not passed from one generation to the next, but occur randomly.

Often the first noticeable sign of Rett's disorder is slowed head growth. Loss of hand coordination and movement follows, replaced by repetitive (stereotypical) hand wringing. Language development and social skills suffer around 2 to 3 years of age. Eventually, physical problems progress so that the child walks with difficulty—often stiffly with jerking. In addition, breathing may become labored and produce a sighing sound. These problems can cause severe disability and sometimes death.