There ’ another aspect furin cleavage site narrows path natural emergence origin even further.As everyone knows ( may least recall high school ) , genetic code uses three units DNA specify amino acid unit protein chain.When read groups 3 , 4 different kinds DNA unit specify 4 x 4 x 4 64 different triplets , codons called.Since 20 kinds amino acid , enough codons go around , allowing amino acids specified one codon.The amino acid arginine , instance , designated six codons CGU , CGC , CGA , CGG , AGA AGG , A , U , G C stand four different kinds unit RNA.Here ’ gets interesting.Different organisms different codon preferences.Human cells like designate arginine codons CGT , CGC CGG.But CGG coronavirus ’ least popular codon arginine.Keep mind looking amino acids furin cleavage site encoded SARS2 genome .