Name the selectable marker that can differentiate recombinants from non-recombinants in a single step based on its ability to produce colour in the presenc eof chromogenic substrate?

lac z' gene or $\beta$ - gal  gene of pUC vectors undergo insertional inactivation if a foreign gene is inserted within this gene. As a result, it gives white colonies.

 If the lac z'  gene is intact, it gives blue colored colonies in the presence of chromogenic substrate -X -gal and inducer IPTG.