Breeding experiments were performed between two female mice heterozygous for the current mutation (Itpr1wt/Δ18) and a male mouse heterozygous for the Itpr1opt mutation (Itpr1wt/opt). This resulted in two litters of mice with a total of four affected Itpr1opt/Δ18 pups (from a total of 15; two of seven from first mating; two of eight from the second mating) with a phenotype indistinguishable from that of the Itpr1Δ18/Δ18 and Itpr1opt /opt mice.