A grant proposal written by the U.S.-based nonprofit the EcoHealth Alliance and submitted in 2018 to the Defense Advanced Research Projects Agency, or DARPA, provides evidence that the group was working " or at least planning to work " on several risky areas of research. Among the scientific tasks the group described in its proposal, which was rejected by DARPA, was the creation of full-length infectious clones of bat SARS-related coronaviruses and the insertion of a tiny part of the virus known as a "proteolytic cleavage site" into bat coronaviruses. Of particular interest was a type of cleavage site able to interact with furin, an enzyme expressed in human cells. Since the genetic code of the coronavirus that caused the pandemic was first sequenced, scientists have puzzled over the "furin cleavage site." This strange feature on the spike protein of the virus had never been seen in SARS-related betacoronaviruses, the class to which SARS-CoV-2, the coronavirus that causes the respiratory illness Covid-19, belongs. The furin cleavage site enables the virus to more efficiently bind to and release its genetic material into a human cell and is one of the reasons that the virus is so easily transmissible and harmful. Many who believe that the virus that caused the pandemic emerged from a laboratory have pointed out that it is unlikely that the particular sequence of amino acids that make up the furin cleavage site would have occurred naturally.