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10 " The researchers looked deeper into these observations, in hopes of gaining insight into the mechanisms underlying the high evolutionary rate and extraordinary immunologic plasticity of influenza HA. They probed in more detail the precise codons that are used by the virus to encode the influenza HA1 protein. The discriminated between codons on the basis of volatility. Each three-nucleotide codon is related by a single nucleotide change to nine 'mutational neighbours.' Of those nine mutations, some proportion change the codon to a synonymous codon and some change it to a nonsynonymous one, which directs the incorporation of a different amino acid into the protein. More volatile codons are those for which a larger proportion of those nine mutational neighbours encode an amino acid change. The use of particular codons in a gene at a frequency that is disproportionate to their random selection for encoding a chosen amino acid is termed codon bias. Such bias is common and is influenced by many factors, but here the collaborators found strong evidence for codon bias that was particular for and restricted to the amino acids making up the HA1 epitopes. Remarkably, they observed that influenza employs a disproportionate number of volatile codons in its epitope-coding sequences. There was a bias for the use of codons that had the fewest synonymous mutational neighbours. In other words, influenza HA1 appears to have optimized the speed with which it can change amino acids in its epitopes. Amino acid changes can arise from fewer mutational events. The antibody combining regions are optimized to use codons that have a greater likelihood to undergo nonsynonymous single nucleotide substitutions : they are optimized for rapid evolution. "

, Viruses: Agents of Evolutionary Invention

11 " I must have roamed dementedly about for a time in the streets. When I at last got back to my own place, Faustine was again there ahead of me, coiled torpid in the bed like a loathsome boa-constrictor. She was already in the never-never land where ghouls like her belonged. I covered her face with one of the pillows, pressed down upon it with the weight of my whole body, held it there until she should have been dead ten times over. Yet when I removed the pillow to look, the black of strangulation was missing from her face. She was still in that state of suspended animation that defied me, a taunting smile visible about her lips.I had a gun in my valise, from years before when I'd been on an engineering job in the jungles of Ecuador. I got it out, looked it over. It was still in good working order, although it only had one bullet left in it. That one would be enough. She wasn't going to escape me! I pressed the muzzle to her smooth white forehead, mid-center. " Die, damn you!" I growled, and pulled the trigger back. It exploded with a crash. A film of smoke hid her face from me for a minute. When it had cleared again, I looked.There was no bullet-hole in her skull!A black powder-smudge marked the point of contact. The gun dropped to the floor with a thud. That ineradicable smile still glimmered up at me, as if to say: " You see? You can't." I rubbed my finger over the black; the skin was unbroken underneath. A blank cartridge, that must have been it. I raised her head; there was a rent in the sheet under it. I probed through it with two fingers. I could feel the bullet lying imbedded down in the stuffing of the mattress.(" Vampire's Honeymoon) "