This confirmed observations from a tumour sequencing study (Labbe et al, 2017), and because plasma DNA captures the mutations coming from different parts of the tumour, we found that in some patients, these TP53 mutations pre‐existed at AF < 1% in plasma prior to treatment and later became the dominant mutations in plasma when patients progressed or exhibited histological transformation to SCLC. Here, TP53 is linked to small cell lung carcinoma.