Rescue cell lines function
Rescue experiments are the gold-standard for confirming that an effect is due to the gene of interest. 

Removing a gene may have off-target effects but if you can return function to 'normal' by supplementing the gene back into the cell-line/animal then this is strong evidence that the gene and its product are the cause of effects that are seen in the experiment. 

Wild-type cells would normally be included to show what 'normal' response  is.