Sp1 and Sp3 bind to the t-PA –7351 site. Nuclear proteins (5 μg) from HUVECs were incubated with the wild-type GC-box probe with or without the addition of antibodies directed against human Sp1, Sp2, Sp3, or Sp4 as indicated. Binding specificity of nuclear proteins with the labeled oligonucleotide was confirmed by addition of excess cold GC-box specific competitor (lane 2) and nonspecific competitor (lane 3). NS indicates nonspecific.