The beta-globin LCR is made up of at least four DNasel hypersensitive sites (5'HS1-5'HS4), which are able to direct position independent, copy number dependent expression in transgenic mice. However, 5'HS3 alone directs high-level, single copy transgene expression in transgenic mice, but only when linked to the beta-globin promoter, the betaIVS2 and the 260-bp 3' beta-globin enhancer. The betaIVS2 contains an ATR detrimental to retroviral production, and also contains sequences that are required for expression at all integration sites and for high-level transcription. These elements include Gata-1 and Oct-1 sites as well as an MAR that contains 2 SatB1 sites. As gamma-globin is a better anti-sickling protein than beta-globin, this study aims to evaluate the ability of five new beta/gamma-globin hybrid cassettes that exclude an AT-rich sequence deleterious for vector production, with respect to their ability to express gamma-globin at optimal levels in transgenic fetal mice. In addition to the transgenic mice, I have evaluated two of these cassettes in an HIV-1 self-inactivating vector, for their ability to produce high viral titer and express in MEL cells. This study demonstrated that the Oct-1 site requires a functional interaction with the betaIVS2 enhancer to provide high expression levels at multi copy, and that the Igmu 3'MAR can substitute for the ATR to rescue single copy expression in beta/gamma-globin transgenic mice. Additionally, for the first time, a beta/gamma-globin cassette that expresses at single copy was produced as high titer lentivirus.