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getting nasty zits. Biogenetics are like that, man. Got to take into account the entire variance of the human genome, you know?"
    "So what about the goldfish?"
    "Right. Lots of people had discovered that using endomorphic tissue as a base provided you with a ready chunk of material you could mutagenetically alter directly. But as a conversion vector it's got a lot of problems - mainly, the body thinks it's a virus.
    "Just about the time I bailed from school some of my peers figured out that they could use cancer cells to create recombinants - cells that combine DNA sequences. The ability to hack cancer cells has been around a long time; you just set it up to create the cells of your choice and off you go. But the body readily identifies those cells as foreign, typically, and kills them. Cancer in the wild propagates because it finds a variety that's particularly virulent - whatever you end up mixing together at home is pretty easily taken care of by the immune system. So what we figured out was a marriage of the two - a genetically unstable cancer that sought endomorphic cells and was vulnerable to conjoining."
    Tonx looked proudly up at Fed, his hands laced behind his head. Fede folded his arms.
    "What the hell does that mean?" he asked.
    Tonx smiled. "It means the cancer cells bust open the endomorphic cells and ingest part of their DNA. Makes a new cell, a combination of them both."
    Tonx watched Fede for a moment, traced the slow steady vector of his thinking and pre-empted him. "The benefit of that is twofold. One, you don't have to try to introduce an entire DNA sequence into the cancer cell before you launch it. That shit's hard, and you usually get something too unstable to last long enough to get it into a new system. Two, the body starts out thinking it's trying to kill two things, and then suddenly those two things are gone and you have a third thing instead. Takes a little while for the body to adapt.
    "The reason folks are so shit-hot to make this stuff work is for healing or replacing limbs or other tissue. If you could just stick a chunk of endomorphic base tissue onto the bloody stump of somebody's arm and have it grow back you'd have yourself some pretty powerful economic leverage.
    "The way they've done it so far is by having the cancer cells look for two types of cells: stem cells, and endomorphic cells. Stem cells are endomorphic, so it's not as hard as it sounds."
    Tonx paused, looked at Fede from the corner of one eye. "Stem cells are the ones that turn into whatever other kind of cells is needed."
    "That's the stuff the breeders are used for, right? They get them out of embryos?" asked Fed.
    "Yeah, but I'm not talking about that. Every body makes its own stem cells, it's part of the normal healing process."
    "Okay" said Fed. "I got it." He was starting to enjoy himself now, the back-and-forth of his brother's stream of thought and his questions. Like old times. He found himself relaxing into learning, looking for holes in the logic, questioning his own knowledge and marking out things for later exploration.
    "Good" said Tonx. "So you set up the cancer cells to ingest the transformative DNA sequence carried by the stem cell. That's the "message" the stem cell has about what it's supposed to turn into when it gets to the damaged part of the body. The cancer cell eats that, combines with the endomorphic tissue, and uses the message sequence from the stem cell to inform the final mutation. The result - if you're lucky - is a cell that's accepted into the body as a replacement for the damaged cell."
    "So does the body accept it as native?" shot back Fed, seizing on a loose thread from earlier in the conversation. "You said that one of the benefits to the process was that the body starts out looking for two types of tissue, and then discovers a third."
    Tonx nodded. "Bingo. The big problem is that no matter what you set up the mutagenic cells - that's the result of all this rigmarole - whatever you set up the