Gray platelet syndrome (GPS) is an autosomal recessive bleeding disorder that is 
characterized by large platelets that lack α-granules. Here we show that 
mutations in NBEAL2 (neurobeachin-like 2), which encodes a BEACH/ARM/WD40 domain 
protein, cause GPS and that megakaryocytes and platelets from individuals with 
GPS express a unique combination of NBEAL2 transcripts. Proteomic analysis of 
sucrose-gradient subcellular fractions of platelets indicated that NBEAL2 
localizes to the dense tubular system (endoplasmic reticulum) in platelets.