Single molecule biology: coming of age.

Cellular heterogeneity and stochastic fluctuation play key roles in biological processes. Single molecule approaches have the key advantage of avoiding ensemble averaging, allowing the observation of transient intermediates and heterogeneity (both static and dynamic). Thus they have revolutionised the way many biological questions are addressed. The challenge ahead is to develop integrated approaches such as the combination of single molecule imaging with single molecule manipulation to probe the dynamics of gene regulatory and cell signalling networks in living cells.