Genetic screening of Saccharomyces cerevisiae mutants defective in Ca2+ homeostasis identified cls2, which exhibits a specific Ca(2+)-sensitive growth phenotype. We describe here the CLS2 gene and a multicopy suppressor (named BCL21, for bypass of CLS2) of the cls2 mutation. The CLS2 gene encodes a polypeptide of 410 amino acid residues, and its hydropathy profile indicates that the predicted Cls2 protein (Cls2p) contains ten putative membrane spanning regions. Immunofluorescent staining of the yeast cells expressing epitope-tagged Cls2p suggests that Cls2p is localized to endoplasmatic reticulum (ER) membrane. A cls2 disruption strain is viable, but shows a Ca(2+)-sensitive phenotype like the original cls2 mutants. BCL21 suppresses the cls2 disruption mutation, indicating that the multicopy suppression does not require the Cls2p. Suppression of cls2 was observed even after introduction of a single-copy plasmid harboring BCL21. The BCL21 gene encodes a protein of 382 amino acid residues and is identical to the SUR1 gene. sur1 was originally isolated as a suppressor of rvs161, which has reduced viability in nutrient starvation conditions. Possible mechanisms of the multicopy suppression are discussed.