The disclosure provides methods for inhibiting the activity of a miRNA cluster in a cell, and also for screening a cell for a phenotype(s) of interest resulting from inhibition of a miRNA cluster. The methods use a cluster pool which comprises at least one miRNA inhibitor specific for each miRNA in the miRNA cluster. MiRNA inhibitors are described that induce apoptosis in breast cancer cells and hence are useful in the treatment of breast cancer. The disclosure also provides pharmaceutical compositions which are useful for the treatment of breast cancer; methods for inducing the nuclear translocation of NF-κB in a breast cancer cell; methods for inducing the nuclear translocation of c-Jun in a breast cancer cell; method for inhibiting the nuclear translocation of NF-κB in a breast cancer cell; and methods for providing prognostic medical information relating to breast cancer progression.