6. Are they transiently expressing tissue stem cell specific surface markers? (e.g., CD34+ for hematopoietic cells [8]; CD133+ for brain cells [12, 13], CD44+, CD33-, LowLin- for breast cells [9-11]; CD20+ for skin cells [40]; CD44+,α 2β 1hi/CD133+ for prostate cancer cells [41]. Here, CD44 is linked to prostate cancer.