Provided are a DNA fragment, a pharmaceutical composition, and the like, which can simplify the period of creating knockout animals. The DNA fragment includes: a detection sequence that codes for a detection marker other than drug resistance (preferably a visually-detectable marker)； a resistance sequence that codes for a resistance marker for a drug that inhibits the proliferation of one or more species of at least prokaryotic organisms； a promoter sequence that is located upstream of the resistance sequence and functions in prokaryotic organisms； and a regulatory sequence that is located upstream of all of the above sequences and, only when inserted into a target region of the genome of at least one species of eukaryotic organism, induces the expression of a sequence located downstream. The detection sequence and the resistance sequence are arranged so as to be capable of action.
