In an ES cell line in which a loxP site previously had been inserted into the Hira gene by homologous recombination, a second loxP site was inserted into the genome between the Gp1bb and Cldn5 genes using a retrovirus. Transient cre-expression in the ES cells resulted in deletion of a 150-kb region from Cldn5 to Hira. (J:110378)