A gene whose expression helps to identify transformed cell is known as

A selectable marker for gene expression helps in identification of a transformed cell from non-transformed cells.

(1) Vector is a carrier DNA molecule, used in cloning experiments to carry and introduce a foreign gene of interest into a selected host organism.

(2) Plasmids are extrachromosomal ringlets of DNA found naturally in some types of bacteria. They confer a special trait to the host cell. Plasmid based vectors are widely used in cloning experiments in microbial biotechnology. 

(3) Structural genes are the regions of DNA that code for either protein or RNA but not regulatory genes. Expression of structural genes is under the control of these regulatory genes.