AB Science SA announced that its clinical development program of masitinib in adult patients with mast cell activation syndrome has been approved by the U.S. Food and Drug Administration. The Investigational New Drug approval letter received from the FDA provides authority to proceed with a Phase II study in patients with severe mast cell activation syndrome. MCAS is a disease caused by inappropriate activation of mast cells, which can lead to mast cell mediator release symptoms with a severity ranging from mild to life-threatening. In this aspect, MCAS is similar to indolent and smoldering systemic mastocytosis however, important differences exist that make MCAS a distinct entity from systemic mastocytosis. In mastocytosis, well-defined mutations result in an aberrant population of mast cells with a marked increased proliferation in tissues, whereas MCAS is driven by greater mutational heterogeneity that is associated with aberrant mast cell activation but only modest increases in mast cell numbers due to reduced apoptosis.