CD274 and head and neck squamous cell carcinoma: Eligibility criteria include: (1) R/M HNSCC, inclusive of cancers that originate in the head and neck region for the phase 1 part of the study; (2) R/M, PD-L1-positive HNSCC of the oral cavity, oropharynx, larynx or hypopharynx with the progression of disease on prior pembrolizumab or nivolumab treatment (monotherapy or chemoimmunotherapy) in the last 6 months for the phase 2 part of the study.