We noted that PD-1 PROG melanoma cell lines commonly expressed effectors and markers of de-differentiation30–32, including accumulation of the receptor tyrosine kinase AXL33 and downregulation of the micropthalmia-associated transcription factor (MITF)34 (7/16; 44% PD-1 PROG cell lines) (Supplementary Figs. 8A, 9). This evidence concerns the gene NTRK1 and melanoma.