MRPL9 (mitochondrial ribosomal protein L9) is a structural component of the mitochondrial large ribosomal subunit essential for mitochondrial translation 1. Beyond its canonical role in translation, MRPL9 functions as an oncogenic driver in multiple cancer types. In lung cancer, MRPL9 is significantly upregulated and associated with poor overall and recurrence-free survival; its knockdown inhibits cell proliferation, sphere formation, and migration through modulation of the c-MYC/ZEB1 axis regulating epithelial-mesenchymal transition 1. In hepatocellular carcinoma (HCC), chromosome 1 amplification-driven MRPL9 elevation drives tumorigenesis with altered mitochondrial function gene expression, and MRPL9 elevation correlates with poor survival 2. MRPL9 serves as a diagnostic and prognostic marker in HCC, with expression incorporated into prognostic models predicting survival and drug resistance 3 4. In papillary thyroid cancer, MRPL9 interacts with GGCT to promote proliferation and migration via MAPK/ERK pathway activation 5. Conversely, in early-onset preeclampsia, MRPL9 downregulation impairs trophoblast migration and mitochondrial function through triggering the mitochondrial unfolded protein response 6. These findings establish MRPL9 as a critical node connecting mitochondrial function to cancer progression and pregnancy complications.