Factors that are likely to influence the outcome of a gene transfer trial for the hemoglobin disorders. Knowledge regarding each component will likely facilitate successful gene therapy of hemoglobin disorders. HSC source, ex vivo cell manipulation and transduction conditions will be important components in determining efficient HSC gene transfer and subsequent engraftment. Patient age and disease-specific factors may influence these parameters. Finally, engraftment will be influenced by the type and amount of pre-transplant conditioning.