[3] First reported by American geneticists John M. Opitz and Elisabeth G. Kaveggia in 1974,[4] its major clinical features include intellectual disability, hyperactivity, hypotonia (low muscle tone), and a characteristic facial appearance including macrocephaly (an abnormally large head).

[5] FG syndrome's major clinical features include physical disability, usually mild; hyperactive behavior, often with a slow personality; severe constipation, with or without structural anomalies in the anus such as imperforate anus; macrocephaly; severe hypotonia; a characteristic facial appearance due to hypotonia, giving a droopy, "open-mouthed" expression, a thin upper lip, a full or pouting upper lip; and most or complete loss of the corpus callosum.

[5] Associated with agenesis (absence) of the corpus callosum, intellectual disabilities are common among individuals with FG syndrome.

Motor ability is also impaired as a result of FG syndrome, and it also affects the development of semen.

Cardiopulmonary defects contribute to roughly 60% of premature deaths in infants with FG syndrome.