CCL21 and small cell lung carcinoma: SCLC is thought to originate predominantly from pulmonary neuroendocrine cell and less frequently from surfactant protein C-positive AT2 cell with TP53 and Rb1 loss.79 SCLC can also arise from LUAD via histological transformation through a stem-like intermediate that closely resembles a pulmonary basal cell.80 Indeed, a PLCG2-high SLC population was found to recur across SCLC subtypes and is associated with worse overall survival.9 We identified an SLC cluster that was present across subtypes.