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Thrombocytopenia and absent radius syndrome (TAR; MIM 274000) is an autosomal recessive, frequently fatal disorder, caused by mutations in RNA-Binding Motif Protein 8A (RBM8A). The main characteristics of TAR are transient bleeding episodes early in life and upper limb abnormalities that vary from absence of radii to aplasia of the ulna and humerus. In severe cases the hands arises directly from the shoulder and may themselves display abnormal structure. Bleeding, which decreases in frequency and severity with advancing age, is caused by thrombocytopenia due to decreased numbers of bone marrow megakaryocytes. Transient leukocytosis has also been noted. Additional skeletal abnormalities may be present. These most commonly include: hip dysplasia, knee subluxation and coxa valga. Extraskeletal manifestations including tetralogy of Fallot, atrial and ventricular septal defects, coarctation of the aorta and genitourinary anomalies have been described. The vast majority of TAR cases are caused by a heterozygous deletion of about 200kb on chromosome 1q21.1, including the RBM8A gene, and one of two low frequency SNPs in the regulatory region of the same gene. In rare instances, one of the two RBM8A regulatory SNPs have been documented to be associated with a different, second compound heterozygous RBM8A null mutation other than the 200kb deletion. The described combinations of these compound heterozygous mutations account for more than 90% of TAR cases. RBM8A encodes the Y14 subunit of the exon-junction complex. This complex performs several essential RNA processing tasks including nuclear export and subcellular localization of transcripts, translational enhancement and nonsense-mediated RNA decay.