The main principles behind how PLLA fillers improve acne scars include the following: [10] (1) initiation of a regenerative signal for collagen synthesis by lactic acid, which is achieved by increasing the levels of tumor growth factor‐β; (2) upregulation of the tissue inhibitor of metalloproteinase 1 signaling pathway, which increases the activity of metalloproteinase inhibitors; (3) stimulation of the activity of prolyl hydroxylase by lactic acid, increasing hydroxyproline levels; and (4) metabolism of lactic acid providing energy for collagen synthesis. The gene discussed is TIMP1; the disease is acne.