In this issue of the Biochemical Journal, Wu et al. describe the characterization of new low-affinity Ca2+ indicators for monitoring Ca2+ levels in both the ER (endoplasmic reticulum) and mitochondria. In contrast with other ER Ca2+ sensors, these indicators emit in the red part of the spectrum, allowing for their use in combination with GFP-based probes and expanding the genetically encoded tool kit for the measurement of Ca2+ inside organelles.

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