“…These methods assume that single cells transit from one cellular state to another in a continuous fashion, and that all necessary cellular states for the process under investigation are sampled with sufficient depth, allowing the ordering of cells along a pseudotime trajectory of cellular progression. This process of 'trajectory inference' has been applied successfully to various imaging (Gut et al, 2015;Herring et al, 2018;Serra et al, 2019), CyTof (Bendall et al, 2014;Setty et al, 2016) and sequencing (Chen et al, 2016b;Guo et al, 2017;Haghverdi et al, 2016;Qiu et al, 2017;Setty et al, 2019;Shin et al, 2015;Trapnell et al, 2014;Weinreb et al, 2018;Wolf et al, 2019) datasets. However, trajectory inference solely on cellular states has its limitations, as reviewed recently elsewhere (Kester and van Oudenaarden, 2018;Wagner et al, 2016;and, also in this issue, Tritschler et al, 2019).…”