For example, while human CCL4L1 and CCL4L2 share 100% sequence identity in the coding regions, a fixed mutation at the intron–exon boundary of CCL4L1 results in the production of aberrantly spliced transcripts (Figure 1B and 1C), and a higher CCL4L1 copy number has been associated with an increased risk of acquiring HIV infection [26] and faster rate of progression to AIDS [16]. Here, CCL4L2 is linked to HIV infectious disease.