MFSD12 (major facilitator superfamily domain containing 12) is a transmembrane transporter protein that mediates the import of cysteine into melanosomes and lysosomes, playing critical roles in both pigmentation and cellular redox homeostasis 1. In melanosomes, MFSD12 facilitates cysteine import required for maintaining normal cystine levels and producing cysteinyldopas used in pheomelanin synthesis, thereby regulating skin pigmentation 1. The protein is essential for embryonic development, as Mfsd12 knockout mice die during organogenesis between embryonic days 10.5-12.5, indicating its critical physiological importance 2. In cancer contexts, MFSD12 is upregulated by breast cancer cells to increase lysosomal cyst(e)ine storage, which maintains glutathione levels and buffers oxidative stress 3. The protein's function is regulated by mTORC1 through direct phosphorylation at residue T254 3. Clinically, MFSD12 overexpression correlates with poor neoadjuvant chemotherapy response and prognosis in breast cancer patients, while promoting proliferation, metastasis, and invasion in hepatocellular carcinoma through modulation of the tumor immune microenvironment 34. MFSD12 also contributes to genetic variants influencing skin pigmentation diversity in African populations 5.