A genetic disorder caused by an extension of the CGG trinucleotide repeat in the FMR1 gene on the X chromosome, resulting in reduced or no production of the FMRP protein, which is essential for normal brain development, the most common inherited cause of intellectual disabilities and a known genetic cause of autism spectrum disorders. Symptoms include developmental delays, learning difficulties, social and behavioral problems, anxiety, hyperactivity, and telltale signs such as an elongated face, large ears, and increased joint mobility. Men usually suffer more severely than women due to the presence of only one X chromosome. There is no cure for the cause, but early care, educational support, and targeted therapies can improve the prognosis.