MTT proliferation assay performed on control and knockout NB4-R1 cells. The MTT assay was performed on the indicated days (X axis) in the absence (− ATRA) or presence (+ ATRA) of 10⁻⁵ mol/L ATRA. The Y axis depicts the optical density (O.D.). The growth curves in the left graph depict untransfected NB4-S1, an ATRA-sensitive NB4 clone, and NB4-R1, the de novo ATRA-resistant clone, from which the knockout subclones were derived. The right graph depicts the growth curves for NB4-R1 cells transfected with the pKJ-1 vector (Control) as a control for insertional mutagenesis and growth in G418 and for one representative knockout subclone (K.O.) that shows a reversal in resistance to growth inhibition by ATRA treatment.