Next-generation sequencing has enabled the screening for a causative mutation in X-linked intellectual disability (XLID). We identified KIAA2022 mutations in two unrelated male patients by targeted sequencing. We selected 13 Japanese male patients with severe intellectual disability (ID), including four sibling patients and nine sporadic patients. Two of thirteen had a KIAA2022 mutation. Patient 1 was a 3-year-old boy. He had severe ID with autistic behavior and hypotonia. Patient 2 was a 5-year-old boy. He also had severe ID with autistic behavior, hypotonia, central hypothyroidism, and steroid-dependent nephrotic syndrome. Both patients revealed consistent distinctive features, including upswept hair, narrow forehead, downslanting eyebrows, wide palpebral fissures, long nose, hypoplastic alae nasi, open mouth, and large ears. De novo KIAA2022 mutations (p.Q705X in Patient 1, p.R322X in Patient 2) were detected by targeted sequencing and confirmed by Sanger sequencing. KIAA2022 mutations and alterations have been reported in only four families with nonsyndromic ID and epilepsy. KIAA2022 is highly expressed in the fetal and adult brain and plays a crucial role in neuronal development. These additional patients support the evidence that KIAA2022 is a causative gene for XLID. (c) 2015 Wiley Periodicals, Inc.