In vivo tumour samples subjected to single-cell RNA sequencing prior to and after PD-1 inhibitor treatment24 also showed a similar tendency; moreover, a marked decrease of CCL20 expression occurred specifically in the cell population identified as malignant melanoma cells (Fig. S3D).23 Next, we analysed K5-SOS mice, which express a dominant form of Son of Sevenless (SOS) in the basal keratinocytes. The gene discussed is KRT5; the disease is melanoma.