BACKGROUND: Despite earlier studies demonstrating in vitro propagation of solid tumour cancer stem cells (CSCs) as non-adherent tumour spheres, it remains controversial as to whether CSCs can be maintained in vitro. Additional validation of the CSC properties of tumour spheres would support their use as CSC models and provide an opportunity to discover additional CSC cell surface markers to aid in CSC detection and potential elimination. METHODS: Primary tumour cells isolated from 13 surgically resected colon tumour specimens were propagated using serum-free CSCselective conditions. The CSC properties of long-term cultured tumour spheres were established and mass spectrometry-based proteomics performed. RESULTS: Freshly isolated CD133 þ colorectal cancer cells gave rise to long-term tumour sphere (or spheroids) cultures maintaining CD133 expression. These spheroid cells were able to self-renew and differentiate into adherent epithelial lineages and recapitulate the phenotype of the original tumour. Relative to their differentiated progeny, tumour spheroid cells were more resistant to the chemotherapeutic irinotecan. Finally, CD44, CD166, CD29, CEACAM5, cadherin 17, and biglycan were identified by mass spectrometry to be enriched in CD133 þ tumour spheroid cells. CONCLUSION: Our data suggest that ex vivo-expanded colon CSCs isolated from clinical specimens can be maintained in culture enabling the identification of CSC cell surface-associated proteins.