D1 and D5 receptors (D1-like receptors) promote intracellular cyclic adenosine monophosphate (cAMP) synthesis, while D2 and D4 receptors (D2-like receptors) inhibit cAMP synthesis.[15] cAMP promotes DA synthesis, so D1 and D5 receptors can increase DA levels to inhibit myopia, and their deficiency may be a risk factor for myopia; D2 and D4 receptors have opposite roles to D2 and D5 in maintaining DA levels and increased D2 and D4 can reduce DA levels to induce myopia, which may be a risk factor for myopia. This evidence concerns the gene MAB21L3 and myopia.