CDKN1B and neoplasm: Because miR-221/222 is a negative regulator of p27kip1, a cell-cycle inhibitor and tumor suppressor [60], and tamoxifen-resistant breast cancer cells display up-regulation of these miRNAs and significant reductions in p27kip1 levels [30], miR-221/222 is thought to regulate tamoxifen sensitivy via direct targeting of p27kip1.