RB1 and breast cancer: In ER− parental and palbociclib-resistant MDA-MB-468 (Rb-deficient) breast cancer cells, DDX also downregulated cyclin E2 and cyclin A2 and unexpectedly restored the presence of the cell cycle repressor Rb, which is an integral part of the cell cycle checkpoint inhibiting the progression past G1 (Figure 1B and Figure 2B).