RRM2 and head and neck cancer: Numerous studies have observed that RRM2 is overexpressed in many cancers, including renal cell carcinoma (Xiong et al., 2021), colorectal cancer (Liu et al., 2013), lung cancer (Jin et al., 2020), bladder cancer (Morikawa et al., 2010), and head and neck cancer (Morikawa et al., 2010), and is regarded as a promoter for cancer progression and therapeutic target (Zhan et al., 2021).