DDX39B is a member of the DEAD-box family of ATP-dependent RNA helicases. DEAD-box proteins are ubiquitously expressed from yeast to humans and perform essential functions associated with mRNA metabolism. DDX39B is also a crucial component of the TRanscription-EXport (TREX) super protein complex, which recent studies have highlighted the important role of its subunits in neurodevelopmental disorders. Here, we describe six individuals from five families, four with novel de novo missense variants in DDX39B, and one carrying an inherited splicing variant, all presenting with mild to severe global developmental delay, congenital hypotonia, epilepsy, short stature, skeletal abnormalities and variable dysmorphic features. 3D molecular modeling predicts these variants would alter protein structure. DDX39B is a conserved gene and Drosophila melanogaster (fruit flies) studies were conducted. We generated a new Hel25E Kozak-GAL4 allele which disrupts the fly gene and allows expression of transgenes. We also generated transgenic DDX39B -reference and variant flies. However, human reference DDX39B when overexpressed ubiquitously leads to lethality but the variants found in the patients do not recapitulate the lethality suggesting that the mutants are loss of function alleles. Blood transcriptomics revealed a significant excess of aberrant splicing events, indicating a disrupted mRNA processing as anticipated from the role of DDX39B in mRNA metabolism. Our human genetic data, coupled with in silico and in vivo data supports that DDX39B is a novel candidate gene in a potential group of disorders named TREX-complex-related neurodevelopmental syndrome. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement The DDD study presents independent research commissioned by the Health Innovation Challenge Fund [grant number HICF-1009-003], a parallel funding partnership between Wellcome and the Department of Health, and the Wellcome Sanger Institute [grant number WT098051]. The research team acknowledges the support of the National Institute for Health Research, through the Comprehensive Clinical Research Network. Those who carried out the original analysis and collection of the Data bear no responsibility for the further analysis or interpretation of the Data. We would like to thank The Society for the Relief of Disabled Children and The Edward and Yolanda Wong Fund for their support. The authors thank the patients and their families for participation without which this study could not be done. MFW, SY, HJB, SJ and OK were partially supported by The BCM Center for Precision Medicine Models grant [U54 OD030165]. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Our local Indiana University Institutional Review Board (IRB# 2005902680) approved the study. This study was approved by the institutional review board of the University of Hong Kong/Hospital Authority Hong Kong West Cluster (UW12-211). For proband D and Family 1 (F1.1, F1.2), written informed consent for the DDD study and reporting of clinical features was obtained including specific consent for photo publication (DECIPHER: 264142 and Family1: ). The DDD study has UK Research Ethics Committee approval (10/H0305/83, granted by the Cambridge South REC, and GEN/284/12 granted by the Republic of Ireland REC). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes DDX39B variants reported herein have been deposited to ClinVar, temporary Submission ID: SCV003843896 for NM\_004640.7:c.109G>T, SCV003843897 for NM\_004640.7:c.275G>A, SCV003843898 for NM\_004640.7:c.368G>A, SCV003843899 for NM\_004640.7:c.132C>G, SCV003843900 for NM_004640.7:c.433-1G>T.