CD34 and head and neck squamous cell carcinoma: In head and neck squamous cell carcinoma, the colony stimulating factor 2 (CSF-2) gene encoding granulocyte-macrophage colony-stimulating factor (GM-CSF) expression, linked to MDSC, was strongly associated with poor overall survival in anti-PD-1 treated recurrent and metastatic patients, and CD34, an MDSC marker, was increased in tumors without derived therapeutic benefit (34).