Immunofluorescence staining confirmed the co-localization of SPHK1 and PD-L1 in the HNSCC cell lines SAS and SCC15 (Figure 1H, I), with a higher proportion of SPHK1+/PD-L1+ cells compared to the normal oral keratinocyte cell line HOK (Figure 1J). Here, SPHK1 is linked to head and neck squamous cell carcinoma.