A dynamic view of cellular processes by in vivo fluorescence auto- and cross-correlation spectroscopy.

Fluorescence correlation spectroscopy (FCS) is becoming increasingly popular as a technique that aims at complementing live cell images with biophysical information. This article provides both a short overview over recent intracellular FCS applications and a practical guide for investigators, who are seeking to integrate FCS into live cell imaging to obtain information on particle mobility, local concentrations, and molecular interactions. A brief introduction to the principles of FCS is provided, particularly emphasizing practical aspects such as the choice of appropriate dyes and positioning of the measurement volume in the sample. Possibilities and limitations in extracting parameters from autocorrelation curves are discussed, and attention is drawn to potential artifacts, such as photobleaching and probe aggregation. The principle of dual-color cross-correlation is reviewed along with considerations for proper setup and adjustment. Practical implications of nonideal conditions including incomplete focus overlap and spectral cross-talk are considered. Recent examples of both auto- and cross-correlation applications demonstrate the potential of FCS for cell biology.