BACKGROUND: Pathogenic variants in genes encoding CaM (calmodulin) are associated with a life-threatening ventricular arrhythmia syndrome (calmodulinopathy). The in vivo consequences of CaM variants have not been studied extensively and there is incomplete understanding of the genotype-phenotype relationship for recurrent variants. We investigated effects of different factors on calmodulinopathy phenotypes using 2 mouse models with a recurrent pathogenic variant (N98S) in Calm1 or Calm2 . METHODS: Genetically engineered mice with heterozygous N98S pathogenic variants in Calm1 or Calm2 were generated. Differences between the sexes and affected genes were assessed using multiple physiological assays at the cellular and whole animal levels. Statistical significance among groups was evaluated using 1-way ANOVA or the Kruskal-Wallis test when data were not normally distributed. RESULTS: Calm1 ^N98S/+ ( Calm1 ^S/+ ) or Calm2 ^N98S/+ ( Calm2 ^S/+ ) mice exhibited sinus bradycardia and were more susceptible to arrhythmias after exposure to epinephrine and caffeine. Male Calm1 ^S/+ mice had the most severe arrhythmia phenotype with evidence of early embryonic lethality, greater susceptibility for arrhythmic events, frequent premature beats, corrected QT prolongation, and more heart rate variability after epinephrine and caffeine than females with the same genotype. Calm2 ^S/+ mice exhibited a less severe phenotype, with female Calm2 ^S/+ mice having the least severe arrhythmia susceptibility. Flecainide was not effective in preventing arrhythmias in heterozygous CaM-N98S mice. Intracellular Ca ²⁺ transients observed in isolated ventricular cardiomyocytes from male heterozygous CaM-N98S mice had lower peak amplitudes and slower sarcoplasmic reticulum Ca ²⁺ release following in vitro exposure to epinephrine and caffeine, which were not observed in cardiomyocytes from heterozygous female CaM-N98S mice. CONCLUSIONS: We report heterogeneity in arrhythmia susceptibility and cardiomyocyte Ca ²⁺ dynamics among male and female mice heterozygous for a recurrent pathogenic variant in Calm1 or Calm2 , illustrating a complex calmodulinopathy phenotype in vivo. Further investigation of sex and genetic differences may help identify the molecular basis for this heterogeneity.