Although there are four amino acid variations of S protein between 2019-nCoV and SARS-CoV, 2019-nCoV can also bind to the human angiotensin-converting enzyme 2 (ACE2), the same host receptor for SARS-CoV, as 2019-nCoV can bind to the ACE2 receptor from the cells from human, bat, civet cat, and pig, but it cannot bind to the cells without ACE211,33–35.