CXCR4 and B-cell chronic lymphocytic leukemia: Replication of in vivo stimuli is crucial for potential applications of these 3D CLL models (e.g., drug testing) as, for example, CXCR4-mediated interactions hold cancer cells within a protective tumor microenvironment and are linked to resistance to therapeutic agents (Philipp-Abbrederis et al., 2015).