The movement of melanocytes into the HF throughout HF development and cycling is controlled by stromal cell-derived factor 1 (SDF-1), also known as C-X-C motif chemokine 12 (CXCL12), and its receptor, C-X-C chemokine receptor 4 (CXCR4). The gene discussed is CXCL12; the disease is hydrops fetalis.