We present the cognitive abilities of females from five families who carry a mutation in a gene (KDM5C, formerly JARIDIC or SMCX) in Xp 11.2 that encodes a transcriptional regulator with histone demethylase activity that is specific for dimethylated and trimethylated H3K4. In this report, the cognitive abilities of females who carry KDMSC mutations are compared to females who carry mutations in other genes known to cause X-linked intellectual and developmental disability (XLIDD) conditions. The KDM5C mutation carriers had higher mean scores on the abstract/visual and quantitative sections of the Stanford-Binet Intelligence Scale: Fourth Edition and lower mean short term memory scores. Implications for counseling are presented.