The inositol 1,4,5-trisphosphate (IP) receptors (IPRs), which form tetrameric channels, play pivotal roles in regulating the spatiotemporal patterns of intracellular calcium signals. Mutations in IPRs have been increasingly associated with many debilitating human diseases such as ataxia, Gillespie syndrome, and generalized anhidrosis. However, how these mutations affect IPR function, and how the perturbation of as-sociated calcium signals contribute to the pathogenesis and severity of these diseases remains largely uncharacterized. Moreover, many of these diseases occur as the result of autosomal dominant inheritance, suggesting that WT and mutant subunits associate in heterotetrameric channels. How the in-corporation of different numbers of mutant subunits within the tetrameric channels affects its activities and results in different disease phenotypes is also unclear. In this report, we investigated representative disease-associated missense mutations to determine their effects on IPR channel activity. Additionally, we designed concatenated IPR constructs to create tetrameric channels with a predefined subunit composition to explore the functionality of heteromeric channels. Using calcium imaging techniques to assess IPR channel function, we observed that all the mutations studied resulted in severely attenuated Ca release when expressed as homotetramers. However, some heterotetramers retained varied degrees of function dependent on the composition of the tetramer. Our findings suggest that the effect of mutations depends on the location of the mutation in the IPR structure, as well as on the stoichiometry of mutant subunits assembled within the tetrameric channel. These studies provide insight into the pathogenesis and penetrance of these devastating human diseases.