PGP and melanoma: In keeping with this, based on studies of 21 primary malignant melanomas and 37 metastases, Schadendorf et al (1995) reported 43% of both primary and metastatic tumours to express MRP-1, with a lack of detectable expression of MDR1/P-gp (except in one primary tumour) suggesting that the chemo-resistant nature of melanoma cells is unlikely to be mediated by MDR1/P-gp.