YAP1 and neoplasm: Furthermore, the tumor suppressor pathway hypo-signaling limits the proliferation of mature stem cells as well as progenitor cells.44 When Hippo signaling is activated, the transcriptional coactivators Yes-associated protein 1 (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are phosphorylated, leading them to stay in the cytoplasm.