Rescue cell lines function I'm reading in a paper in which the authors have made three cell lines to study the function of a gene of interest ( _Mbd3_ ): The one is knocked out for the gene of interest [ _Mbd3_ -/-], the other is floxed for the one allele [ _Mbd3_ fl/-], and the third is rescued knock out cell line [ _Mbd3_ -/- (rescued)], that means that in the cell line of knocked out cells they have stably transfected a plasmid expressing the gene of interest. I am then wondering, why they use this rescue cell line, instead of using the wild type cells with the wildtype alleles of _Mdb3_?

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.