FNIP1 is a critical regulator of cellular metabolism and immune homeostasis that integrates nutrient sensing with mitochondrial function 1. As a binding partner of the GTPase-activating protein FLCN, FNIP1 controls mTORC1 signaling by promoting FLCN recruitment to lysosomes and interaction with Rag GTPases 2. In low-amino acid conditions, FNIP1 inhibits FLCN's GTPase activity, inactivating mTORC1 and promoting nuclear translocation of transcription factors TFEB and TFE3, which induce lysosomal and autophagy genes 3. Upon amino acid restimulation, FNIP1 disassembly liberates FLCN activity, reactivating mTORC1 3. Beyond mTORC1 signaling, FNIP1 acts as an HSP90 co-chaperone, inhibiting its ATPase activity to activate client proteins 4. FNIP1 also functions in the reductive stress response, with its reduced cysteine residues recognized by the CRL2(FEM1B) ubiquitin ligase for proteasomal degradation when mitochondrial activity is suppressed 56. FNIP1 is essential for B-cell development 7. Mutations in FNIP1 cause Immunodeficiency 93 with hypertrophic cardiomyopathy, associated with immunological abnormalities and metabolic perturbations 1. Recent evidence links FNIP1 to muscle-bone cross-talk and adipose tissue thermogenesis 89.