ACTN1 and Macrothrombocytopenia: Of these, ACTN1, which encodes alpha-actinin 1, was regarded as the most likely candidate by virtue of its functional role in the cytoskeleton and by analogy to some of the known macrothrombocytopenia genes, such as MYH9 [2,3], FLNA [4] and TUBB1 [6].