he opened the shortest of the texts and scanned some of the captions to graphics : `` a plasmid harboring a normal loxp has little chance of remaining integrated in a genome at a similar loxp site since the cre recombinase will eliminate the integrated dna fragment ... '' `` lentiviral vectors injected into one-cell embryos or incubated with embryos from which zona pellucida was withdrawn were particularly ... '' `` a more efficient way to replace a gene relies on the use of mutant es cells devoid of the hprt gene ( hypoxanthine phosphoribosyl transferase ) .