The human transcription factor Sp1 binds upstream of certain viral and cellular promoters and activates initiation of RNA synthesis from these promoters by RNA polymerase II. The Sp1 binding sites of both simian virus 40 and a related monkey promoter contain multiple copies of the sequence GGGCGG, three or four of these sequences forming close contacts with Spl. The clustered contacts fall on one strand of the DNA and are arranged similarly in the major groove of the DNA helix in both promoters.