PIK3CA and neoplasm: Tumor-derived mutations (e.g., TP53, PIK3CA, FAT1, NOTCH1) are detectable in matched saliva from oral cavity squamous cell carcinoma (OSCC) with high tissue–saliva concordance (>70–80%), and ultra-deep, error-corrected sequencing improves detection of low-frequency variants and subclones [16,17,19,25].