Abstract: Protein modification by glycosylation occurs through an essential biochemical pathway that produces mannosyl side chain substrates, which are covalently attached to proteins in the endoplasmic reticulum. We used DNA microarray analysis to characterize the cellular response to a conditional defect (pmi40-101) in the protein glycosylation pathway. Expression profiles were obtained from DNA microarrays containing essentially every gene from Saccharomyces cerevisiae. We validated the microarray analysis by examining the expression patterns of induced genes using transcriptional lacZ fusions. The major class of genes differentially expressed in the glycosylation mutant overlapped significantly with that of a starvation response and included those required for gluconeogenesis, the tricarboxylic acid and glyoxylate cycles, and protein and amino acid biosynthesis. Two mitogen-activated protein (MAP) kinase pathways were also activated in the mutant, the filamentous growth and protein kinase C pathways. Taken together, our results suggest that a checkpoint is activated in response to a protein glycosylation defect, allowing the cell to mount an adaptive response by the activation of multiple MAP kinase pathways.