Since levels of matrix metalloproteinases (MMP)-2 and MMP‐9 have been found to be increased in patients with LAM in serum and lung biopsies [33, 34], doxycycline, which inhibits the production and activity of several MMPs, has also been investigated as a therapeutic agent; however, a randomized, double-blind, placebo-controlled trial revealed no differences between doxycycline and placebo in the rate of FEV1 decline and quality of life [35]. This evidence concerns the gene MMP9 and lymphangioleiomyomatosis.