ETR101, a human homolog of rat pip92, is a cellular immediate early gene induced by extracellular stimuli such as serum growth factors. ETR101 encodes a short-lived, proline-rich protein (ETR101) exhibiting no significant sequence similarity to any other known protein, and little is known about its function. We investigated the functioning of ETR101 as a transcriptional activator for the gene ISYNA1, which encodes human inositol 1-phosphate synthase. We constructed a yeast strain in which the chromosomal region of the PSS1 promoter was replaced with the human ISYNA1 promoter. Using this yeast strain, we screened human cDNAs, which activated the ISYNA1 promoter, and thus expressed the PSS1 as a reporter gene. We obtained two types of cDNA, E2F1, known as a gene encoding Rb-binding protein, and ETR101. The E2F1 gene product (E2F1) is known to bind to and activate the ISYNA1 promoter. In a manner similar to E2F1, ETR101 binds to and activates the ISYNA1 promoter.