a, A t-SNE plot of a 28-d fibroblast-to-iEP reprogramming time course experiment, with time point information projected onto the plot (n = 85,010 cells). b, Overlay of CellTag expression, broken down by CellTag library version, onto the t-SNE plot in a. Before the initiation of reprogramming, fibroblasts were transduced with the CellTagV1 library. Three days after initiation of reprogramming, cells were transduced with the CellTagV2 library. Finally, 13 d after the start of the reprogramming process, cells were transduced with the CellTagV3 library. Following this scheme, CellTag expression is detected in 99% of cells, with almost 70% of cells expressing two or more CellTags to support confident cell tracking.