The RED light is on! New tools for monitoring Ca²⁺ dynamics in the endoplasmic reticulum and mitochondria

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.