POU2F3 and neoplasm: Using a new liquid biopsy assay, it is demonstrated that SCLC subtypes, as determined by patient tumor tissue staining and cell lines, can be accurately identified by measuring the mRNA expression of subtype transcription factors (ASCL1, POU2F3, and NEUROD1) in circulating exosome‐rich extracellular vesicles (Exo).