Recently, mutations in the EPO gene itself have been identified in relation to familial erythrocytosis whereby a single base pair deletion in exon 2 caused a frameshift that truncated translation of the main EPO messenger RNA (mRNA), but converted a typically noncoding mRNA, which was transcribed from an alternative promoter within intron 1, to produce excess functional EPO mainly through the liver [60]. Here, EPO is linked to primary familial polycythemia due to EPO receptor mutation.