CD34 and metachromatic leukodystrophy: Retargeted FVs can be produced at high titer and efficiently transduce human CD34+ cells suggesting they will be useful for gene therapy of SCID-X1, chronic granulomatous disease, thalassemias, and potentially metabolic diseases that can be treated by hematopoietic stem cell (HSC) transplantation such as metachromatic leukodystrophy.