CXCR4 and B-cell chronic lymphocytic leukemia: In addition, it was revealed that according to CXCR4 expression in combination with CD5 (a surface molecule characteristically expressed on CLL B cells), CLL clones can be defined into subgroups varying in the time elapsed since the last cell division (also called “age”): the newly originated, proliferative fraction (PF; CXCR4DimCD5Bright), the double dim fraction (DDF; CXCR4DimCD5Dim), the intermediate fraction (IF; CXCR4IntCD5Int), the double bright fraction (DBF; CXCR4BrightCD5Bright) and the resting fraction (RF; CXCR4BrightCD5Dim) (188–190).