HPRT1 and infection: In this case a single gene, Csn3, was utilized as the anchoring point by targeting it with a vector containing the loxP site, the 5’ Hprt and the neomycin resistance gene followed by an infection with retrovirus in order to introduce another loxP site, with the second half of Hprt gene and puromycin resistance gene, for selecting infected cells with this antibiotic.