Schematics of the predicted ice structure in the prestellar core stage (left) and the observed water deuterium fractionation toward embedded low-mass protostars (right). The outer layer of the ice mantle, which has high water D/H ratio, may stem from nonthermal desorption in the cold envelope or cold gas-phase formation. Meanwhile, the whole mantle is sublimated in the hot corino, lowering the HDO/H₂O ratio. The schematical structure is proposed by Furuya et al. (2016). The figure is adopted from van Dishoeck et al. (2021).