Most individuals with disease causing defects in the FOXP1 gene show a delayed development during the first years of life, and often intellectual disability when older. Mostly, the delay in speech and language development is more pronounced than the delay in acquiring motor skills. Autistic features, including repetitive behaviour, are frequent, but most individuals do not meet the necessary criteria for an definite diagnosis of autism.
Other features seen in some affected individuals include cardiac defects, undescended testes, frequent infections, strabismus and refractive error, floppiness (low muscle tone), abnormalities of the kidneys and genitourinary tract, seizures, and some degree of hearing loss.
Most individuals with disease causing defects in the FOXP1 gene may show specific facial characteristics, e.g. they may show a higher forehead, downward slanting eyelids, and fuller lips.
FOXP1-related developmental problems are rare, but it exact frequency is unknown. The condition affects boys and girls, with equal severity. Nevertheless, the condition is highly variable and may cause severe intellectual disability in one person, and (near) normal intelligence in another. Most patients have moderate intellectual disability (IQ~50). The cause of this clinical variability in severity is currently not understood.