Computer graphics representation of haemoglobin, the oxygen-carrying molecule of blood, constructed during research for a blood substitute at the U.K. Medical Research Council (MRC) Laboratory of Molecular Biology, Cambridge. Haemoglobin's four protein chains are shown in blue & purple, with the four haem groups, the oxygen-binding sites, in red. MRC scientists used recombinant DNA techniques on the bacterium Escherichia coli, to synthesize human haemoglobin. The need to develop a blood substitute has arisen because of concern over contamination of stocks by pathogens such as HIV, the virus causing AIDS.