To investigate the significance of this, we generated two PRMT1 and PRMT5-knockdown MCF7 cell lines each expressing distinct short hairpin RNA (shRNA) sequences (shPRMT1[1] and shPRMT1[2]; shPRMT5[1] and shPRMT5[2]; Figure 1C) and analyzed their ability to form mammospheres, a measure of the number and proliferative potential of tumor-initiating cells in vitro (Ponti et al., 2005). The gene discussed is PRMT5; the disease is neoplasm.