CSF2 and Schnyder corneal dystrophy: The HU-mediated reduction in leukocyte count may occur via one or more pathways (Fig 7): the generation of NO which induces apoptosis in leukocytes (pathway 1) [57]; the inhibition of DNA synthesis through the suppression of ribonucleotide reductase (pathway 2) [58]; or mediation by a reduction in granulocyte macrophage colony-stimulating factor (GM-CSF), which has a role in leukocytosis in SCD (pathway 3) [43].