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The paternally expressed extra-large stimulatory G protein gene (XLαs) is a splice variant of the stimulatory G-protein gene (Gsα) consisting of XL-exon1 and exons 2–13 of Gsα. A second open reading frame (ORF) in XL-exon1, that completely overlaps the XL-domain ORF, encodes ALEX, which is translated from the XLαs mRNA and binds the XL-domain of XLαs. We previously demonstrated that a paternally inherited functional polymorphism in XL-exon1, consisting of a 36 bp insertion and two nucleotide substitutions, is associated with Gs hyperfunction in platelets, leading to an increased trauma-related bleeding tendency and is accompanied by neurological problems and brachydactyly in two families. Here, we describe eight additional patients with brachydactyly, who inherited the same XLαs polymorphism paternally and who show Gs hyperfunction in their platelets and fibroblasts. All carriers also have an elongated ALEX protein, as a consequence of the paternally inherited insertion. The in vitro interaction between the two elongated XLαs and ALEX proteins is markedly reduced. Moreover, XLαs or ALEX can be co-immunoprecipitated with an antibody against either ALEX or XLαs in platelets from a control but hardly from patients with the XLαs/ALEX insertion. In contrast to the strong interaction between the two wild-type proteins, we suggest that this defective association results in unimpeded receptor-stimulated activation of XLαs. The paternally inherited double XLαs/ALEX functional polymorphism is also associated with elevated platelet membrane Gsα protein levels. Both phenomena contribute to increased Gs signaling in patients with platelet hypersensitivity towards Gs-agonists and may be accompanied by neurological problems or growth deficiency.