MMP1 and head and neck squamous cell carcinoma: Detailed information of mutation sites, types and frequencies of MMP1 are shown in Fig. 4B. Missense mutation of MMP1 was the most common form (82/94, 87.23%) and P412S/H/L mutation in the Hemopexin domain was detected in 2 cases of skin cutaneous carcinoma (SKCM), 1 case of lung adenocarcinoma (LUAD) and 1 case of head and neck squamous cell carcinoma (HNSC) (Fig. 4B), which may result in frame-shift mutation of the MMP1, translation from S (serine) to H (histidine)/L (leucine) at the 412 site of MMP1 protein and promoting protein truncation.