## 4 Discussion
Especially, CASP8 in HNSC is a well-known cancer-related gene included in the Cancer Gene Census whose somatic mutation is predominantly observed in oral squamous cells. Although a very low \(p\)-value was not achieved in this survival analysis, the CASP8 gene plays an important role in programmed cell death, and many studies have demonstrated that this gene is associated with tumor prognosis and immune signatures [53; 54]. Similarly, it has been reported that the BPTF gene, detected in LUSC, is related to prognosis and survival in lung adenocarcinoma, non-small cell lung cancer [55; 56], and tumor microenvironments [57]. Moreover, TTN identified inOV is the longest gene in the human genome, and is thus usually excluded from cancer genome analyses because of its much higher chance of being mutated. The gene with the second highest PCC in OV was MACF1, contributing to tumor progression [58]; its clinical importance and relationship to survival of the gene was recently discovered in serious ovarian cancer [59]. Thus, it would be proven that our approach was reliable to search the mutational signatures related to immunity and survival outcome. Eventually, the study might potentially be applied in clinical applications to improve patient survival. Tumor development and progression are associated with the accumulation of the mutations, but not all the sequence alterations cause tumors. Therefore, the identification of the driver mutations and the finding of their functional roles are some of the major problems in tumor research. However, the mutational signatures did not clearly explain the acquisition of a specific driver mutation according to the mutation-causing processes. Nevertheless, a recent study explored how the molecular influences of the passenger mutations and the mutational signatures are related to the molecular functional impact [60]. Likewise, my experimental results showed the functional aspects of the mutational signatures, particularly related to immune activities, even though I did not handle the tumor driver genes directly. Moreover, on the other hand, some studies have attempted to analyze the connection between the mutational processes and the gain of the driver mutations by positive selection during cancer evolution [61; 62]. By utilizing the links of the mutational signatures and the driver or passenger mutations from these kinds of studies, it would be possible to enhance the knowledge of tumor initiation, evolution, and immune activity. A limitation of this study was that it did not consider molecular features or clinical information such as tumor grade, histology, and lymph node involvement. For instance, breast tumors were subdivided into luminal A, luminal B, basal-like, Her2-enriched, etc. By subgrouping the samples based on oncological features or molecular properties, subsequent analysis might obtain informative functional characteristics of the mutational signatures. Thus, in future works, in-depth investigations on a single cancer type should be carried out to uncover the relationships between the mutational signatures and the tumor infiltration. To fully grasp the potential of mutational signatures, additional studies are required to develop a mechanistic understanding of how mutational signatures form. Further integrative analyses using multi-omics data and clinical information may provide novel information or insights to help understand the roles of mutational signatures in tumor biology. For instance, it might be possible to understand why distinct mutational signatures are related to different prognoses and immune reactions and can help predict patient responses to drugs or immunotherapy by combining various clinical data. Moreover, it is also necessary to determine how mutational signatures are related to previously identified clinical and molecular biomarkers. Furthermore, in future works, in-depth investigations on a single cancer type should be carried out to uncover the relationships between the mutational signatures and the tumor infiltration.