Since stem cells can continually self-regenerate, making more stem cells, and differentiate into many different specialized cell types, they play an important role in our development and health. But there can also be a dark side -- stem cells can sometimes become cancer stem cells, proliferating out of control and leading to blood cancers, such as leukemia and multiple myeloma. The self-renewing nature of cancer stem cells makes them particularly hard to eradicate, and they're often the reason a blood cancer reoccurs.