Fig. 2.

Cre-loxP-mediated chromosomal engineering in mice. A loxP site (arrow) is targeted into the first endpoint of the engineered segment (blue) in the embryonic stem (ES) cell genome with a positive selectable marker, such as neo (the neomycin-resistance gene; N). Next, a second loxP site is targeted to the other endpoint with another positive selectable marker such as puro, the puromycin resistance gene (P). A Cre expression vector is then transferred by electroporation into double-targeted ES cell clones. If two loxP sites are targeted onto the same chromosome homologue and oriented in the same direction (cis), recombination between the sites will lead to a deletion (Df; A). If two loxP sites are targeted onto two separate homologues and oriented in the same direction (trans), the recombination will lead to a duplication (Dp) and the reciprocal deletion (Df) (B).