a, Somatic mutations identified in NOTCH3 and NOTCH4 are mapped to the protein domains. Damaging mutations are highlighted in red. Mutations found in murine SCLC tumours are highlighted in blue. b, Quantification of tumour lesions and per cent tumour area to lung in TKO (n = 5) and TKO;N1ICD (n = 4) mice 3 months after Ad-Cre instillation. Statistical significance was determined by two-tailed unpaired Student’s t-test. c, Representative immunohistochemistry for GFP or tdTomato in lungs from TKO;Rosa26mT/mG mice approximately 6 months after tumour induction. Left scale bar, 500 µm; right and middle: scale bar, 50 µm. d, Representative immunostaining for Notch2 in lungs from TKO;Rosa26N2ICD mice approximately 6 months after tumour induction. Left scale bar, 500 µm; right scale bar, 50 µm. e, Quantification of the per cent recombination at the Rosa26 locus in TKO;Rosa26mT/mG (n = 6) and TKO;Rosa26N2ICD mice (n = 10; two-tailed unpaired Student’s t-test). f, Cell viability assay of the human SCLC cell line NJH29 transfected with a N1ICD (Notch1) expression plasmid or empty vector control (Ctrl) (3 independent biological replicas with 3 technical replicas each). Fold growth was normalized to day 0; representative images were taken on day 6. Scale bar, 50 µm. g, Immunohistochemistry staining in FFPE embedded tissues of TKO and TKO;N2ICD mice. Scale bar, 50 µm. h, Quantitative RT–PCR validation of Notch1 induction and the expression of common Notch target genes after N1ICD transfection in murine SCLC cells (three biological replicas; two-tailed paired Student’s t-test). i, Mouse SCLC cells transfected with control or N1ICD (Notch1) were analysed 48 h later by gene expression microarrays. Gene Set Enrichment Analysis (GSEA) was performed on these data; selected significant gene sets are displayed. j, k, EdU analysis of mouse (j) and human (k) SCLC cells (three independent biological replicas with three technical replicas each; two-tailed paired Student’s t-test). *P < 0.05; **P < 0.01; ***P < 0.001. Data are represented as mean ± s.d.