Knockout of STUB1 using two independent sgRNAs led to a marked increase in CTSL protein levels in HNSCC cell lines HN8 and FaDu (Figure 4A), whereas CTSL mRNA levels remained unchanged (Figure S2H), indicating that STUB1 negatively regulates CTSL at the post‐transcriptional level. This evidence concerns the gene CTSL and head and neck squamous cell carcinoma.