A novel gene, IRE1, of Saccharomyces cerevisiae was cloned through genetic complementation of a myo‐ inositol auxotrophic mutant. The predicted amino acid sequence indicated that IRE1 encodes a protein of 126983 Da with two highly hydrophobic regions, probably a signal sequence and a membrane‐spanning region. The carboxy‐terminal region of IRE1 showed close sequence similarity to the catalytic domains of protein kinases. Disruption of the IRE1 locus caused myo‐inositol auxotrophy. The IRE1 product is very likely a protein kinase required for myo‐inositol synthesis.