Intellectual disability/mental retardation occur in 2-3% of newborns in the general population. In 80% of the cases, the causes remain elusive. Improved molecular and cytogenetic techniques have enabled the detection of small chromosome abnormalities, which could explain the clinical features in mentally retarded individuals. The aim of this study was to perform a detailed chromosome analysis of a female patient with unexplained intellectual disability, seizures and delayed psychomotorical development. The patient’s chromosomes were analyzed by G-banding revealing an unbalanced translocation of chromosome X and 22. In addition, an isodicentric chromosome appeared in approximately 38% of the patient’s leucocytes. aCGH detected a 45.4 Mb deletion on one chromosome X, and the X-inactivation test revealed a highly skewed inactivation (96:4). Signs and symptoms of mild Turner Syndrome are usually seen in patients with partial deletion of a large region of the X chromosome. The symptoms in the patient analyzed within this thesis, however, did not match those expected in Turner Syndrome. FISH was used to map der(X) to search for potentially disrupted genes at the translocation breakpoint, and to determine the origin of the isodicentric chromosome. RNA sequencing suggested that the normal X chromosome was active and that der(X) was inactive in 96% of the patient’s leucocytes, respectively. A possible outcome from der(X) inactivation is the spread of the X-inactivation onto the translocated 22q material. SNP array was used to assess whether the patient was suffering from a recessive disease.