Bloom syndrome (BS) is a rare monogenic, autosomal recessive disorder characterized by pre-postnatal growth deficiency, short stature, sun-sensitive facial rash and and an greatly increased risk of developing cancers at early age. Additionally, cells from BS patients display increased sister chromatid exchanges (SCEs) as a key cytogenetic hallmark.In addition to BLM, 2018, Martin et al reported biallelic pathogenic TOP3A mutations in 10 patients who showed prenatal growth restriction, microcephaly, dwarfism and elevated SCEs, which are Bloom syndrome-like. TOP3A belong to the conserved type IA subfamily of human topoisomerases with TOP3B. TOP3A contains only DNA activity and binds to BLM as a part of BTRR complex to resolves double Holliday junctions (dHJs) arising from repair of DNA breaks, while TOP3B is the only topoisomerase have dual DNA-RNA activity and decreases cellular R-loops by forming TOP3B-DDX5 cleavage complex or by unwinding negatively supercoil. Dysfunction of TOP3B has been linked with accumulation of cellular R-loops in different models (Saha S et al, 2022) .In this study, we used human embryonic stem cell (hESC)-derived cerebral organoid (hCO) models (Lancaster et al, 2013) as well as 2D differentiation methods (Zhang et al, 2010; Shi et al,2012), we showed that lack of TOP3B causes a dramatic size reduction in hCOs. Our results suggested that TOP3B is a novel regulator of NSCs in mammalian neocortical development and could be a potential etiological factor in human brain developmental malformations.