Upstream binding factor (UBF) is a transcription factor required for expression of the 18S, 5.8S, and 28S ribosomal RNAs, along with SL1 (a complex of TBP (MIM 600075) and multiple TBP-associated factors or 'TAFs'). Two UBF polypeptides, of 94 and 97 kD, exist in the human (Bell et al., 1988 [PubMed 3413483]). UBF is a nucleolar phosphoprotein with both DNA binding and transactivation domains. Sequence-specific DNA binding to the core and upstream control elements of the human rRNA promoter is mediated through several HMG boxes (Jantzen et al., 1990 [PubMed 2330041]).