Recent progress in mammalian gene targeting has now advanced the scope of site-specific alteration of genomic DNA from the realm of mouse embryonic stem (ES) cells to that of human somatic cells. 1–3 The implications of these advances for development of novel gene therapy strategies were discussed in Paris, in October 1997 at the ‘Gene Therapy: The Next Generation’meeting. Of the potential strategies discussed at the meeting, modification of genomic loci by gene targeting appears to have the most immediate therapeutic application. In particular, the targeting of genomic DNA with RNA/DNA chimeric oligonucleotides or with small DNA fragments has made important strides. 1, 3 The use of gene targeting approaches over the cDNA-based gene therapy strategies has several advantages. The most relevant is that the relationship between the coding and regulatory elements of the targeted gene is maintained. Only by maintaining this level of genomic integrity can one be assured that the targeted gene will be appropriately expressed in an individual cell. Such cellappropriate expression will be critical both for long-term maintenance and for ensuring that there is no additional pathology that may be the result of long-term cell-inappropriate expression. Another major advantage of recently developed gene targeting strategies has been the relatively high efficiency of targeted replacement. 1–3 Generally, efficiencies between 1 and 10% have been observed. When considering the treatment of diseases where 100% corrected function is not necessary to restore normal phenotype, these gene targeting strategies have a distinct appeal.

Site-specific alteration of genomic sequences by small fragments of DNA, ie small fragment homologous replacement (SFHR), has shown promise for treatment of cystic fibrosis (CF) 1, 4–6 and may have direct applications to gene therapy of other inherited disorders. 1 The target locus for SFHR of CF was a 3-bp deletion in exon 10 of the CF transmembrane conductance regulator (CFTR) gene that results in deletion of phenylalanine at codon 508 (ΔF508). SFHR-mediated replacement has been used to correct transformed4–6 CF airway epithelial cells. In addition to the insertion of 3-bp, SFHR has proven effective for site-specific deletion. The ΔF508 sequence has been introduced into primary normal human airway epithelial cells. 4 Moreover, studies in mouse ES cells and mouse airway epithelial cells4, 7, 8 have given rise to isogenic cell clones carrying ΔF508 CFTR. These studies indicate that at least 3-bp can be deleted or inserted in a