Abstract: Yeast cells in an isogenic population do not all display the same phenotypes. To study such variation within a population of cells, we need to perform measurements on each individual cell instead of measurements that average out the behavior of a cell over the entire population. Here, we provide the basic concepts and a step-by-step protocol for a recently developed technique enabling one such measurement: fluorescence in situ hybridization that renders single mRNA molecule visible in individual fixed cells.CI - Copyright (c) 2010 Elsevier Inc. All rights reserved.