8. Representative formalin-fixed paraffin-embedded (FFPE) tumor specimens (blocks preferred) or at least 15 unstained slides, with an associated pathology report, for testing at the study sponsor site and determined to be evaluable for tumor PD-L1 expression prior to study enrolment; patients with fewer than 15 unstained slides available at baseline (but no fewer than 10) may be eligible following discussion with Merck representatives.