Our results showed that aside from IDO-mediated anti-tumour immune activity, CSCC malignant progression altered the level of miR-142-5p in serum exosomes, indicating that serum exosomal miR-142-5p may discriminate between indolent and aggressive CSCC and contribute to the development of personalised diagnostic strategies for patients with different progression risks. Here, IDO1 is linked to neoplasm.