SPHK1 and head and neck cancer: Our results are consistent with previous studies demonstrating an association between increased SPHK1 expression and aggressive oncogenic behaviors such as larger tumor, deeper invasion depth, advanced clinical stage, poorer pathologic differentiation, higher invasive capacity, and/or chemotherapeutic resistance in head and neck cancer [25], thyroid cancer [26], salivary duct cancer [22], esophageal cancer [27], colorectal cancer [28], and bladder cancer [29].