I know some people doing research in this field, and from what I understand, it's fairly well-established that there are such cells. However, what isn't known is the exact mechanism and any dependence on tumor type. Often these are isolated later on, so not much is known about the nascent capabilities of these cells. Basically, a lot of people think they exist and there's evidence corroborating this, but not a whole lot else is known, and what is may not be widely applicable across cancers and tumor types.