The ease with which HbF production can be increased in subjects with SS disease was a totally unexpected phenomena on the basis of previous models of HbF synthesis. The fact that these drugs induce rapid increases in F cell production and increase HbF per F cell and HbS in non-F cells, may be important clues as to the mechanism of the action of these drugs. It is far from clear whether either one of these agents will be eventually used as therapy for subjects with SS disease. The more recent problems with cytotoxicity illustrated by HU, and the potential carcinogenic effect of 5-aza limit our ability to perform large controlled clinical trials at this time. The observation that HbF production is increased by two agents which perturb DNA replication may be important clues in understanding not only the origins of differential gamma versus beta globin gene expression but also the mechanism by which HbF is restricted to expression in only certain cells during normal erythroid maturation.