Highly conserved TREX-mediated mRNA export is emerging as a key pathway in neuronal development and differentiation. TREX subunit variants cause neurodevelopmental disorders (NDDs) by interfering with mRNA export from the cell nucleus to the cytoplasm. We have previously reported genetic, molecular and predicted protein structure evidence implicating four X-chromosome linked THOC2 gene variants in intellectual disability (ID). We have now identified an additional twelve affected individuals from ten unrelated families with three de novo and nine maternally inherited pathogenic or likely pathogenic variants in THOC2 extending the genotypic and phenotypic spectrum. These comprise three rare missense THOC2 variants that affect evolutionarily conserved amino acid residues and reduce protein stability, and two canonical splice-site THOC2 variants that result in C-terminally truncated THOC2 proteins. We have also performed clinical assessment and molecular studies on a de novo variant in a female with an epileptic encephalopathy and discuss an additional four families with rare THOC2 variants with supportive evidence for pathogenicity. Severe neurocognitive features, including movement and seizure disorders, were observed in this cohort. THOC2 is component of a highly conserved multi-subunit TREX (Transcription-Export) complex that besides its canonical function in exporting mRNAs from the nucleus to the cytoplasm is involved in diverse and critical roles in gene expression, 3’ mRNA processing, stress responses, mitotic progression and genome stability as well as developmental processes such as pluripotency maintenance and haematopoiesis. Taken together, the data suggest that even subtle alterations to the canonical molecular pathways such as mRNA export, otherwise essential for cellular life, can be compatible with life, but lead to NDDs in humans.