Recent experimental evidence has shown that a heterozygous C→G transversion at nucleotide 77 in exon 4/A of the gene encoding CD45 results in a defective CD45 alternative splicing, with altered expression of its isoforms, which has been associated with increased susceptibility to MS (144, 145) (Fig. 1). The gene discussed is PTPRC; the disease is myeloid sarcoma.