Transcription factor. Inhibits cell proliferation by activating either CDKN1A/p21 transcription or RB1 transcription. . [Isoform 1]: Binds CDKN1A promoter and activates its transcription; this activity is further potentiated in the presence of EP300 (synergistic) and ZBTB17/Miz-1 (additive). . [Isoform 3]: Activates RB1 transcription most probably by antagonizing ZBTB17 repression of RB1. Does not bind directly RB1 promoter. . This is the function of ZBTB49 (zinc finger and BTB domain containing 49, ENSG00000168826).