MRPL18 (mitochondrial ribosomal protein L18) functions primarily as a mitochondrial ribosomal component and import factor for cytosolic 5S rRNA. In its precursor form, MRPL18 exhibits RNA chaperone activity, folding 5S rRNA into an import-competent conformation recognized by rhodanese (TST) 1. The protein binds to helix IV-loop D of 5S rRNA and facilitates its mitochondrial import 1. Beyond mitochondrial ribosome assembly, MRPL18 has unexpected cytosolic stress-responsive functions. Under cellular stress, MRPL18 generates a cytosolic isoform via alternative translation that incorporates into 80S ribosomes, facilitating selective translation of heat-shock proteins during the stress response 2. Additionally, parkin-mediated regulation of MRPL18 affects mitochondrial morphology; parkin deficiency causes MRPL18 accumulation in the cytosol, where it competitively binds Drp1, inhibiting mitochondrial fission and promoting hyperfusion in Drosophila muscles 3. Clinically, MRPL18 dysregulation associates with multiple diseases. Type 2 diabetes elevates MRPL18 in colonic mucosa, contributing to a field cancerization phenotype that increases colon cancer risk 4. Elevated MRPL18 expression promotes breast cancer progression through PI3K pathway activation and correlates with reduced immunotherapy efficacy 5. MRPL18 also associates with white matter hyperintensities and reduced survival in non-small cell lung cancer 6, 7, suggesting broader pathological relevance.