SPHK1 and head and neck squamous cell carcinoma: We then validated SPHK1 expression at the mRNA and protein levels in clinical samples of head and neck squamous cell carcinoma (HNSC), stomach adenocarcinoma (STAD), and liver hepatocellular carcinoma (LIHC) using RT‐qPCR and immunohistochemistry, and assessed its effect on cancer cell viability using the CCK‐8 assay.