Which gene  is inserted as marker in screening recombinant transformants from non-recombinant transformants in  a plasmid vector?

Gene z can be used as second marker instead of antibiotic resistance marker gene to identify the recombinant transformants which appear as white colonies from non-recombinant blue colonies, because the gene inserted in lac z region of vector cannot form active β-galactosidase that can convert the X-gal substrate into blue colored product and the colonies of non-recombinants appear blue.