Witteveen-Kolk syndrome (OMIM 613406) is a recently defined neurodevelopmental syndrome caused by heterozygous loss-of-function variants in *SIN3A*. We define the clinical and neurodevelopmental phenotypes related to *SIN3A*-haploinsufficiency in 28 unreported patients. Patients with *SIN3A* variants adversely affecting protein function have mild intellectual disability, growth and feeding difficulties. Involvement of a multidisciplinary team including a geneticist, paediatrician and neurologist should be considered in managing these patients. Patients described here were identified through a combination of clinical evaluation and gene matching strategies (GeneMatcher and Decipher). All patients consented to participate in this study. Mean age of this cohort was 8.2 years (17 males, 11 females). Out of 16 patients ≥ 8 years old assessed, eight (50%) had mild intellectual disability (ID), four had moderate ID (22%), and one had severe ID (6%). Four (25%) did not have any cognitive impairment. Other neurological symptoms such as seizures (4/28) and hypotonia (12/28) were common. Behaviour problems were reported in a minority. In patients ≥2 years, three were diagnosed with Autism Spectrum Disorder (ASD) and four with Attention Deficit Hyperactivity Disorder (ADHD). We report 27 novel variants and one previously reported variant. 24 were truncating variants; three were missense variants and one large in-frame gain including exons 10–12.

Meena Balasubramanian, Alexander J. M. Dingemans, Shadi Albaba, Ruth Richardson, Thabo M. Yates, Helen Cox, Sofia Douzgou, Ruth Armstrong, Francis H. Sansbury, Katherine B. Burke, Andrew E. Fry, Nicola Ragge, Saba Sharif, Alison Foster, Annachiara De Sandre-Giovannoli, Sahar Elouej, Pradeep Vasudevan, Sahar Mansour, Kate Wilson, Helen Stewart, Solveig Heide, Caroline Nava, Boris Keren, Serwet Demirdas, Alice S. Brooks, Marie Vincent, Bertrand Isidor, Sebastien Küry, Meyke Schouten, Erika Leenders, Wendy K. Chung, Arie van Haeringen, Thomas Scheffner, Francois-Guillaume Debray, Susan M. White, Maria Irene Valenzuela Palafoll, Rolph Pfundt, Ruth Newbury-Ecob, Tjitske Kleefstra