Somatic gene therapy, as generally conceived, involves reconstituting a biological function by adding a normal gene to somatic, i.e. non-germline, cells which are genetically deficient in that gene product (Friedmann, 1989). Single gene or Mendelian disorders which are recessive (autosomal or X-linked) are particularly attractive candidates for such a therapeutic approach since successful introduction of one normal gene copy would be expected to prevent the pathological phenotype. Moreover, as has been noted for many inborn errors of metabolism, even relatively low levels of residual function (5–10% of normal) may prevent the major clinical pathology associated with complete deficiency (Ledley, 1990). Thus, full restoration of physiological levels of deficient gene product, while desirable, may not be necessary for the clinical efficacy of any particular gene therapeutic strategy.