ECP is mainly produced during the terminal expansion of the eosinophils in the bone marrow Eosinophil progenitors (EoP's) in the bone marrow are the first cell identified exclusively of the eosinophil lineages. These EoP's express the cell surface markers IL-5R + CD34 + CD38 + IL-3R + CD45RA -, haematopoietic lineage associated transcription factor GATA-1, ECP mRNA transcripts and have visual characteristics of early eosinophilic blast cell [9, 10] . Most of the granule protein production takes place as EoP's undergo the final stages of maturation [11, 12] . ECP is synthesised, transported and stored in the mature secondary granules at such a high rate as that when the eosinophils are ready to leave the bone marrow, they contain 13.5 μg ECP/10 6 cells [13] ( Figure 1B ). Eosinophils are the major ECP producing cell while monocytes and myelomonocytic cell lines produce minute amounts in comparison [14] . Activated [15] but not resting neutrophils also produce some ECP and have the ability to take up further ECP from the surrounding environment storing it in their azurophil granules [16, 17] . In the myelo-eosinophilic cell line HL-60 clone 15, ECP production is dependent on a nuclear factor of activated T-cells (NFAT)-1 binding site in the intron of the ECP gene (denoted RNASE3) [18] . The RNASE3 gene was formed by gene duplication of an ancestral gene about 50 million years ago, the other duplication gene product being the eosinophil granule protein EPX/EDN gene (RNASE2). ECP and EPX/EDN are two ribonucleases with such a high degree of homology that they are unique to humans and primates and not found in other species. After this gene duplication however, ECP lost part of its ribonuclease activity, but acquired cytotoxic activity, whereas EDN/EPX remained a potent ribonuclease [19] .