PDCD1 and melanoma: The DHMC coding department was contacted to identify patients 18 years of age or older who have been diagnosed with advanced malignant melanoma (Stage IIIC, IV) between January 1st 2011 and December 15th 2017, and who had received ipilimumab, nivolumab, and pembrolizumab (or investigational anti-PD-1) without concurrent use of metformin (cohort A), or with concurrent use of metformin (cohort B).