Because FMR1 sits on the X chromosome, boys and men — who have only one X chromosome — tend to be more consistently and more severely affected than girls and women, who have a second X chromosome that can partly compensate.

• Developmental and intellectual features. Delayed speech and motor milestones are usually the first sign in early childhood. Intellectual disability in affected boys is common, typically in the mild to moderate range, while affected girls often have milder or no intellectual impairment.
• Behavioral and emotional features. Attention difficulties, hyperactivity, anxiety, and difficulty tolerating changes in routine are frequent. Many children avoid eye contact, and autism spectrum disorder is diagnosed in roughly half to two-thirds of boys with the full mutation.
• Physical features. These often become more noticeable with age and can include a long face, prominent forehead and ears, flexible joints, and flat feet. After puberty, affected males frequently have enlarged testicles.
• Medical features. Low muscle tone, frequent ear infections, reflux, crossed or lazy eyes, seizures, and a heart valve finding called mitral valve prolapse can occur.

Diagnosis is confirmed with a blood test that directly counts the CGG repeats in the FMR1 gene and checks whether the gene has been chemically switched off (methylation analysis). This is different from a standard chromosome test and must be specifically requested. Testing is recommended for any child with unexplained developmental delay, intellectual disability, or autism spectrum disorder, and for family members of someone already diagnosed.