To prove this, by focusing on the tumor nest ROIs across patients, more than 30% of patients (13/43) had varying molecular subtypes as defined via conventional subtyping markers ASCL1, NEUROD1, and POU2F3 at ROI level even within the same tumor cores, highlighting the existence of ITH of SCLC. The gene discussed is POU2F3; the disease is small cell lung carcinoma.