Yeast kre mutants define a pathway of cell wall (1-*6)-jp-D-glucan synthesis, and mutants in genes KRES and KRE6 appear to interact early in such a pathway. We have cloned KRES, and the sequence predicts the product to be a large, hydrophilic, secretory glycoprotein which contains the COOH-terminal endoplasmic reticulum retention signal, HDEL (1->3)-P-D-glucan and a (1->6)-P-D-glucan (12). The (1->3)-,B-D-glucan is the most abundant polysaccharide, contains some 1,500 glucose residues, and is composed of repeating units of linear (1->3)-13-linked residues, with some 3% of these branched through (1->6)-p3-linkages (20). The (1->6)-,-D-glucan is a smaller, highly branched molecule containing approximately 140 glucose residues. This glucan is largely connected through linear (1--6)-,-linked units, though it contains some linear (1->3)-1 linkages and a high proportion, some 14%, of (1->3, 1->6)-linked branched residues (21).(1-+6)-P-D-Glucan serves as a cell wall receptor for the yeast Kl killer toxin protein, and the binding of the toxin to the cell wall appears essential for toxin action (1, 6). Mutants have been isolated which are resistant to killer toxin (killer resistant) and which have reduced levels of (1->6)-P-Dglucan (15; Boone et al., in press). Such mutants define three genes, KREJ, KRE5, and KRE6, whose components are required for the sequential assembly of (1-+6)-P-D-glucan.The KREI gene codes for a secreted threonine-and serinerich agglutinin-like protein necessary for the addition of (1--6)-,-linked outer chains to a mixed linked core glucan structure (Boone et al., in press). Mutants