MMP1 and head and neck squamous cell carcinoma: Immunofluorescence staining of tumor tissues from five patients with HNSCC showed that the protein expression levels of SPHK1 and MMP1 were higher in patients 1, 3, and 4, and lower in patients 2 and 5, whereas the protein expression levels of CD8a and GZMB were lower in patients 1, 3, and 4, and higher in patients 2 and 5 (Figure 3G).