The emerging roles of Mst1/Mst2 in lymphocyte trafficking, adhesion, and cell polarity are distinct from the canonical Hippo-LATS-YAP pathway to restrain cell proliferation and are consistent with phenotypes of MST1 mutations identified in human immunodeficiencies with recurrent infection and autoantibody production (35). This evidence concerns the gene MST1 and immunodeficiency disease.