SLC16A4 encodes monocarboxylate transporter 4 (MCT4), a plasma membrane protein mediating carboxylic acid transmembrane transport, particularly lactate efflux from cells. MCT4 functions as a lactate exporter crucial for cellular metabolic homeostasis across multiple tissues. Mechanistically, MCT4 regulates intracellular lactate-pyruvate balance and extracellular lactate accumulation. In cancer cells, MCT4 prevents intracellular acidification by excreting lactate produced through glycolysis 1. Beyond transport, MCT4-mediated lactate efflux influences histone lactylation, a post-translational modification affecting gene transcription 2. Clinically, MCT4 dysfunction contributes to multiple diseases. In heart failure, increased MCT4 expression drives lactate export and metabolic rewiring toward hypertrophy 3. In type 2 diabetes, abnormal MCT4 upregulation causes excessive cardiomyocyte lactate efflux, triggering oxidative stress and inflammation in diabetic cardiomyopathy 4. In cancer, MCT4 expression promotes tumor aggressiveness and metastasis 5, while intratumoral lactate accumulation via MCT4 impairs anti-tumor immunity 6. MCT4 inhibition represents a therapeutic target, improving outcomes in cancer immunotherapy 6, atherosclerosis 2, diabetic cardiomyopathy 4, and potentially synergizing with other drugs 1.