Transcription factor which plays a key role in the Hippo signaling pathway, a pathway involved in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. The core of this pathway is composed of a kinase cascade wherein MST1/MST2, in complex with its regulatory protein SAV1, phosphorylates and activates LATS1/2 in complex with its regulatory protein MOB1, which in turn phosphorylates and inactivates YAP1 oncoprotein and WWTR1/TAZ (By similarity). Binds m-cat elements from muscle-specific promoters and differentially activate transcription. Isoform B has probably a transactivation capacity that is lacking in the other isoforms. Isoform D may be defective in DNA binding. Enriched in cardiac and skeletal muscle. Was originally called TEF-1, but is the ortholog of mammalian TEF-3.