The main difference between the two tests is their resolution. The resolution of conventional karyotyping or cytogenetic analysis is limited and can only detect changes in the number and/or structure of chromosomes larger than 5-10 million base pairs (Mb). The resolution of array CGH or molecular karyotyping is much higher, as it allows the simultaneous detection of very small alterations (gains or losses of DNA fragments) responsible for more than 300 genetic syndromes associated with mental retardation, autism, heart disease and other pathologies, most of which are not detectable by conventional karyotyping.