M utations in Ca²⁺-handling proteins in the heart have been linked to exercise-induced sudden cardiac death. The best characterized of these have been mutations in the cardiac Ca²⁺ release channel known as the ryanodine receptor type 2 (RyR2). RyR2 mutations cause “leaky” channels, resulting in diastolic Ca²⁺ leak from the sarcoplasmic reticulum (SR) that can trigger fatal cardiac arrhythmias during stress. In this issue of the JCI, Song et al. show that mutations in the SR Ca²⁺-binding protein calsequestrin 2 (CASQ2) in mice result not only in reduced CASQ2 expression but also in a surprising, compensatory elevation in expression of both the Ca²⁺-binding protein calreticulin and RyR2, culminating in premature Ca²⁺ release from cardiac myocytes and stress-induced arrhythmia (see the related article beginning on page 1814). In the context of these findings and other recent reports studying CASQ2 mutations, we discuss how CASQ2 influences the properties of Ca²⁺-dependent regulation of RyR2 and how this contributes to cardiac arrhythmogenesis.