Activation of telomere maintenance is a key event in malignant transformation and most frequently conferred by induction of the reverse transcriptase telomerase encoded by TERT. Our previous studies demonstrated that spontaneous regression and maturation occur frequently in Tert-deficient neuroblastoma and also in a Tert-deficient SCLC mouse model, however, the mechanisms underlying these phenomena have remained uncertain. Here we will investigate the molecular mechanisms underlying spontaneous regression in telomerase-deficient and -proficient murine SCLC tumor samples, with a special focus on the cell death mechanisms involved in this process.