Trajectory analysis of epithelial cells (hepatocytes and cholangiocytes) from both healthy and COVID-19 livers (Methods, Fig. 3c) suggests a differentiation path from HEP3 cells, a cell population with the highest pathway activities related to cell replication and expressing WNT and NOTCH signaling pathway genes (e.g., TCF7L1, TCF7L2, FZD6, RBPJ, NOTCH2; [26]) to the highly differentiated HEP2 cells, through HEP4, 1, and 5 intermediates, with HEP6 and HEP7 cell populations directly derived from HEP2. The gene discussed is TCF7L2; the disease is COVID-19.