| Disease ID | Source | Name | Description |
| 300912 | OMIM | Mental retardation, X-linked 98 (MRX98) | A disorder characterized by significantly below average general intellectual functioning associated with impairments in adaptive behavior and manifested during the developmental period. MRX98 patients show delayed psychomotor development, absent or poor speech development, and postnatal growth retardation, often with microcephaly. Some patients show autistic behavioral features, such as stereotypic hand movements and repetitive behaviors. Additional, more variable features include spasticity, axial hypotonia, seizures, drooling, gastroesophageal reflux, and lack of sphincter control. The disease is caused by variants affecting the gene represented in this entry. A chromosomal aberration involving NEXMIF is found in patients with severe mental retardation. Pericentric inversion inv(X)(p22.3;q13.2) with P2RY8 leading to inactivation of NEXMIF (PubMed:15466006). MRX98 transmission pattern is consistent with X-linked recessive inheritance (PubMed:23615299). In some cases, de novo heterozygous loss-of-function mutations have been found in affected females, while some female carriers are asymptomatic (PubMed:26576034, PubMed:27358180, PubMed:27568816). The female phenotype partially overlaps with the reported male phenotype but includes epilepsy as a relevant feature. The variability of disease manifestation in female carriers is probably due to skewed X inactivation with differential expression in the brain (PubMed:26576034, PubMed:27358180, PubMed:27568816). |