AbstractLoss of long-term haematopoietic stem cell function (LT-HSC) hampers the success ofex vivoHSC gene therapy and expansion procedures, but the kinetics and the mechanisms by which this occurs remain incompletely characterized. Here through time-resolved scRNA-Seq, matchedin vivofunctional analysis and the use of a reversiblein vitrosystem of early G1arrest, we define the sequence of transcriptional and functional events occurring during the firstex vivodivision of human LT-HSCs. We demonstrate that contrary to current assumptions, loss of long-term repopulation capacity during culture is independent of cell cycle progression. Instead it is a rapid event that follows an early period of adaptation to culture, characterised by transient gene expression dynamics and constrained global variability in gene expression. Cell cycle progression however contributes to the establishment of differentiation programmes in culture. Our data have important implications for improving HSC gene therapy and expansion protocols.