Kleefstra syndrome (KLEFS, OMIM #610253, #617768) is a rare condition characterized by heterozygous genomic deletions at chromosome 9q34.3 removing the EHMT1 gene or EHMT1 point mutations (KLEFS1), or pathogenic variants in KMT2C on chromosome 7q36.1 (KLEFS2), mostly de novo. This evidence concerns the gene KMT2C and Kleefstra syndrome.