The VSG protein coat is not thought to serve any function beyond acting as a barrier to immunoglobulins. However, this enormous protein is highly immunogenic and exposed to the immune system throughout the life cycle in the form of densely packed homodimers. To maintain a chronic infection the parasite uses an extreme form of antigenic variation involving a large repertoire of several hundred full length vsg genes and many more pseudogenes and gene fragments arrayed throughout the chromosomes and minichromosomes, from which new mosaic genes can be generated through through gene conversion. Expression is limited to vsg genes at one of ~20 BSEs (bloodstream expression sites), only one of which is active at any time, mediated by an ESB (expression site body). Variant switching occcurs at 10^-2 events per cell per generation and is mediated by intragenic recombination into the VGS from silent sites. Hierarchical expression imposes a semi-structured probabilisitic switching mechanism. Telomeric genes are the most likely to be expressed, followed by chromosomal internal genes, and then incomplete genes.