Human immunodeficiency virus (HIV) and its clinical syndrome, acquired immune deficiency syndrome (AIDS), are one of the world's most prominent health problems. To understand the mechanisms underlying HIV transcription and thereby its propagation, we have focused on the molecular interactions at the GC-rich binding sites of the HIV-1 core promoter, a region important for HIV-1 transcription. Previous biochemical studies have shown that Sp1, a zinc finger transcription factor initially isolated as a cellular factor binding that binds to the SV40 early promoter GC-rich sequence, binds to the HIV-1 GC-rich binding sites due to sequence similarities. However, the HIV-1 GC-rich binding sites are considerably different from the Sp1 consensus binding sequence, and recent genetic studies have shown the lack of regulation by Sp1 in numerous genes thought to be regulated by that factor in the past. We reasoned that other factors may bind to the HIV-1 GC-rich binding sites. Using the native HIV-1 GC-rich binding sequence as the bait, genetic screening for interacting factors was performed by the yeast one-hybrid method. A cDNA encoding a novel zinc finger protein named GBF, GC-rich sites binding factor, was isolated from a human peripheral blood leukocyte library. Primary structure analysis of GBF revealed a C2H2 Krüppel-type zinc finger at its C-terminus, and putative acidic and proline-rich domains at its N-terminus. We also show that GBF belongs to a subgroup of Krüppel-type zinc fingers distinct from Sp1. By directly addressing interactions at the HIV-1 GC-rich binding sites, our present study sheds new light on molecular interactions at the HIV-1 promoter.