From OMIM
Opitz-Kaveggia syndrome (OKS) is an X-linked recessive mental retardation syndrome characterized by dysmorphic features, including relative macrocephaly, hypertelorism, downslanted palpebral fissures, prominent forehead with frontal hair upsweep, and broad thumbs and halluces. Most have hypotonia, constipation, and partial agenesis of the corpus callosum. Some patients have sensorineural hearing loss and joint laxity evolving into joint contractures. Affected individuals tend to be hyperactive and talkative (summary by Graham et al., 1999). In their original family, Opitz and Kaveggia (1974) named the disorder 'FG syndrome' according to the Opitz system of using initials of patients' surnames. Genetic Heterogeneity of FG Syndrome Other forms of 'FG syndrome' were characterized due to the similar clinical features observed by Opitz and Kaveggia (1974). FGS2 (300321) is caused by mutation in the FLNA gene (300017) on chromosome Xq28 and FGS4 (300422) is caused by mutation in the CASK gene (300172) on chromosome Xp11. FGS3 (300406) has been mapped to Xp22.3, and FGS5 (300581) to Xq22.3. Risheg et al. (2007) suggested that the designation Opitz-Kaveggia syndrome be reserved for those cases with mutation in the MED12 gene. In part this is justified by the fact that a MED12 mutation was found in the family originally reported by Opitz and Kaveggia (1974). See also Lujan-Fryns syndrome (309520), an allelic disorder with an overlapping phenotype.  http://www.omim.org/entry/305450

From MedlinePlus Genetics
Additional features seen in some people with FG syndrome include widely set eyes (hypertelorism), an upswept frontal hairline, and a large head compared to body size (relative macrocephaly). Other health problems have also been reported, including heart defects, seizures, undescended testes (cryptorchidism) in males, and a soft out-pouching in the lower abdomen (an inguinal hernia).

FG syndrome is a genetic condition that affects many parts of the body and occurs almost exclusively in males. "FG" represents the surname initials of the first family diagnosed with the disorder.

FG syndrome affects intelligence and behavior. Almost everyone with the condition has intellectual disability, which ranges from mild to severe. Affected individuals tend to be friendly, inquisitive, and hyperactive, with a short attention span. Compared to people with other forms of intellectual disability, their socialization and daily living skills are strong, while verbal communication and language skills tend to be weaker.

The physical features of FG syndrome include weak muscle tone (hypotonia), broad thumbs, and wide first (big) toes. Abnormalities of the tissue connecting the left and right halves of the brain (the corpus callosum) are also common. Most affected individuals have constipation, and many have abnormalities of the anus such as an obstruction of the anal opening (imperforate anus). People with FG syndrome also tend to have a distinctive facial appearance including small, underdeveloped ears; a tall, prominent forehead; and outside corners of the eyes that point downward (down-slanting palpebral fissures).  https://medlineplus.gov/genetics/condition/fg-syndrome