of this cell type in the blood (20,21). Expanding CTCs ex vivo is thus necessary for reproducible examination of their genomic makeups and behaviors in vitro in culture or in vivo as patient-derived xenografts (PDXs) (22). CTC-PDX versus PDX. With conventional PDX modeling, pieces of patient tumor are implanted directly to athymic mice for tumor formation (23-25). Conventional PDX suffers from inherent drawbacks (26-29) including extremely low tumor formation rates in mice and less tumor progression