Retinoblastoma protein in cancer onset and progression

So I've been looking at an exam question asking about deregulated transcription factors in cancer onset and progression; one of the more obvious choices for this is the p53 transcription factor but is the retinoblastoma protein also a transcription factor? I'm aware the retinoblastoma (Rb) protein binds to the E2F family of transcription factors in order to regulate transcription of E2F target genes, but is the Rb protein itself a transcription factor or just a co-regulator?