All organisms have an inbuilt protective mechanism -shielding them from attack of viruses - called the immune system. The immune system in the case of vertebrates is highly evolved because it can recognise and eliminate 100 million different substances. According to Martin Gellert, a biochemist at the National Institutes of Health near Washington DC, US, this versatility is derived from genes stolen from an ancient retrovirus, which allows the immune system to reshuffle antibody genes. This reshuffling involves fragmentation and rearrangement of DNA strands till the right combination for elimination of the foreign body is achieved (New Scientist, VoI 149, No 2022).