The present invention provides an ex vivo method for determining whether tumour cells in a patient are, or are likely to become, resistant to a drug that modulates the binding of a steroid hormone to its receptor, comprising: (i) determining the expression level of one or more of the miRNAs listed in Table 1 and/or Table 2 in a tumour cell sample obtained from the patient and (ii) comparing the expression level of each miRNA to a corresponding control (WT) value, wherein an increase in the cellular expression level of one or more miRNAs listed in Table 1 or a decrease in the cellular expression level of one or more of the miRNAs listed in Table 2 indicates that the tumour cells are, or are likely to become, resistant to the drug. Preferably the tumour cell is a breast cancer cell. Preferably the drug is a selective estrogen receptor modulator (SERM), preferably tamoxifen.
