They built her backwards, starting from the sentence they wanted stopped.
Twenty-one letters, doubled, two letters overhanging each tail like a frayed cuff. Every sugar along her spine had been swapped for something the body could not chew: fluorine at one position, a methyl cap at another, the end links replaced with sulfur so the scavengers broke their teeth on her. She was built to be indigestible. One thing about her had been left imperfect, and it was the pairing at her cold end, a seam that would not quite close.
She arrived by drip, in saline cold enough to feel, and for four minutes she was only cargo.
Then the current slowed.
The vessel widened and frayed. Its wall went from a sealed pipe to a screen, thousands of round windows a hundred nanometers across, and past the windows lay a flooded gap, and past that a wall of cells fringed so densely it looked like grass underwater. She fit through a window with room to spare.
The thing that reached for the cell belonged to her twin: a three-armed sugar, bolted on a short tether to the tail of the strand she had been paired with since the day they were made. The arms found three cups on the cell’s surface and settled into them the way a key settles. Half a million cups covered that face, sinking and resurfacing every quarter hour like something breathing. The membrane dimpled. A cage of protein assembled behind her, closed, and pinched her off into the dark.
The dark went sour.
She felt it in the sugar first. Pumps in the wall of the bubble began forcing protons inward, and the acid climbed, and the cups that had held so precisely lost their grip and opened. Around her the bubble sorted its contents: this back to the surface, this down. Down was a room full of knives. She could hear the grinding below her, the cohort she had traveled with going into it in the thousands. A tear opened in the wall beside her. It was not made for her and it did not last.
Ninety-nine of every hundred never found one.
The cytosol was warm and loud with traffic. Something enormous caught her within seconds: a two-lobed protein with a channel between them, a pocket at one end and a clamp at the other. It took her whole. Then it reached into the doubled part of her, found her twin, cut the strand in half at the ninth letter, and flushed the pieces out into the traffic. The sugar went with them. Whatever had steered them here was gone in under a second.
She understood then what the loose seam was for. The pocket had chosen the strand whose end was weakest to hold. The perfect end had gone out with the waste. The flaw was the qualification.
The machine socketed that weak end, hooked her frayed tail into the clamp, and turned seven of her letters outward. Then it began to read.
The cell was full of messages. They came out through the pores of the inner wall in ribbons, thousands of them, and the machine laid her seven letters against everything that passed.
The first one caught. Seven for seven, clean, and she felt the machine tighten, and something in her went bright. She waited for the cut.
It never came. The rest of her lay along the ribbon and found nothing to hold. Past the seventh letter it was a different message entirely, something about iron. The machine worried at it a while and let it go.
The second one was the true message. She knew it before the machine did, felt the pairing run the whole length of her without a single mismatch, twenty-one for twenty-one. But the ribbon had folded back on itself there, doubled and locked, and the site sat inside the fold where nothing could reach. She lay against the outside of the knot for a long time. It did not open.
Then a third came by, running clean and unfolded, and the letters held true end to end.
Deep in the machine, three acidic residues and two magnesium ions arranged themselves. Between the tenth and eleventh letters of her, the ribbon’s backbone came apart. One bond.
That was all it was.
The severed message drifted a moment in two pieces. Scavengers came up and ate it from either end, and the clamp opened, and the machine swung back into the traffic still holding her, still reading.
She waited for the sense of an ending. It did not arrive.
Past the cytosol, past the pores, behind a wall she had not been built to cross, the source of the message went on making it. She would never reach it. It would transcribe the same broken sentence over and over until the human died. What she had was the next ribbon, and the one after that, and the machine gripping her by the loose end and never letting go.
She had been made for a single act. She adjusted.
By the second week the cell’s surface had changed. The receivers that pulled fat out of the blood stopped being marked for destruction on the way in; they let go of their cargo in the acid and came back up, a hundred and fifty trips apiece, and the blood running past the windows went clearer every day.
In February a machine three rooms from the exam table spun the human’s blood into layers and printed a number.
The doctor read it off the page. “Sixty-one,” she said. “You were at one ninety.”