Methods of modifying genes in eukaryotic cells

A method introducing a modification into a genomic exon in the genome of an embryonic stem (ES) cell, comprising:

• (a) introducing an intron into a genomic exon by homologous recombination such that a mature mRNA transcript produced from a genomic region of the genome comprising the genomic exon does not contain the modification, wherein the intron comprises at least one site-specific recombination site;
• (b) introducing a site specific recombination site into a region outside of the genomic exon by homologous recombination, wherein the site-specific recombination site in the intron and the site-specific recombination site in the region outside the genomic exon are oriented in the opposite direction with respect to one another to allow for inversion of the DNA sequence between them following site-specific recombination;
• (c) introducing an embryonic stem cell comprising the genomic exon containing the modification into a blastocyst;
• and
• (d) introducing the blastocyst of (c) into a surrogate mother for gestation of offspring having the modified genomic exon.