ACTN1 and Macrothrombocytopenia: Given that ultra-rare variants in FLNA (the gene encoding filamin A) have recently been reported to result in fragile platelets and macrothrombocytopenia (24), and variants in the actin–cross-linking protein actinin 1 (α-actinin, ACTN1) are one of the most frequent causes of isolated thrombocytopenia with altered platelet morphology (25–27), we examined the levels of these 2 proteins.