The yeast transcription factor complex, Hap2/3/4/5p, is involved in the regulation of many cellular processes, including mitochondrial function and nutrient signaling. Understanding the biology of this transcriptional complex may be useful in studying the human homolog, nuclear factor-YA/B/C, which has recently been implicated in human cancer and diabetes. Haptamide B, a small molecule probe, was discovered to bind a subunit of the yeast transcription complex (Hap3p) by a Small Molecule Microarray screen (SMM). Haptamide B binds Hap3p with a KD of 330nM (SPR), and an IC50 of 23.8 µM (GDH1-lacZ reporter assay). This small molecule probe selectively inhibits Hap2/3/4/5p mediated transcription in vivo, as shown by whole-genome transcription profiling. Koehler et al., J. Am. Chem. Soc., 125, 8420-8421, 2003.