CXCR5 and cancer: In addition, the hypothesis that an externalisation of receptor CXCR5 is restricted to certain phases of cancer development in vivo is also supported by the findings of Meijer et al (2006) who did not detect CXCR5 expression on a variety of tumour cell lines cultured in vitro but who found this chemokine receptor to be expressed on the surface of the same cells several days after injection into mice.