To understand whether CD5 state switching was likely to occur continuously (a common event, randomly distributed over a phylogeny of CD5high and CD5low BCR sequences) or as a rare event (at a single point in CLL evolution, distributed along single lineages of CD5high or CD5low BCR sequences), we determined phylogenetic pattern of the CD5high and CD5low BCR sequences and the probability that an overlap of unique BCR sequences between the CD5low and CD5high samples can happen by chance after a single CD5 state-switching event. This evidence concerns the gene CD5 and B-cell chronic lymphocytic leukemia.