Deleterious effects of calcium indicators within cells; an inconvenient truth.

The study of cellular Ca2+ signalling is indebted to Roger Tsien for the invention of fluorescent indicators that can be readily loaded into living cells and provide the means to measure cellular Ca2+ changes over long periods of time with sub-second resolution and microscopic precision. However, a recent study [1] reminds us that as useful as these tools are they need to be employed with caution as there can be off-target effects. This article summarises these recent findings within the wider context of confounding issues that can be encountered when using chemical and genetically-encoded Ca2+ indicators, and briefly discusses some approaches that may mitigate against misleading outcomes.