,36 Patients with indolent systemic mastocytosis and clonal mast cell activation disorders have an immunophenotypic profile similar to that observed in activated mast cells (i.e. overexpression of CD63, CD69 and CD203c), in addition to high forward and sideward light scatter characteristics, positivity to CD25 and high expression of CD2. The gene discussed is CD63; the disease is systemic mastocytosis.