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A very quiet Friday afternoon. All essential work already done, I sit back with a cup of tea, headphones and Beck's masterful 'Sea Change' CD. Nothing much to say, just killing time.

Killing time – that is a harsh expression. It implies that certain times are worthless, spent without aim or purpose. Just because there is no immediate task at hand, does this work time deserve such an appelation?

It does not. Hands-off time is time to think. To reconsider what I have done, what I need to do. Right now, I am engaged in trying to find out why a certain biochemical reaction is not working for us. A very well-characterized reaction that fails only occasionally. When it does, it is usually because one of the component chemicals has degraded or become contaminated. So, methodically, I replace each component with fresh material and see if this solves the problem.

Right now, I am at the final stage of this process. Final, because I had to specifically order and receive freshly synthesized short fragments of DNA – the primers for this polymerase chain reaction (PCR) – and they did not arrive until yesterday. In the meantime, I was able to substitute for all the other ingredients. No success there.

Nor did I really expect there to be. The DNA primers for this reaction are the most likely component to fail, degrading with time. How effectively the primers will resist this process largely depends on how well cleaned they have been after synthesis – a variable I cannot control, unless I select an expensive and not cost-effective purification process before they are shipped to me.

So now the new primers are here. Redissolved in distilled water and added to a fresh reaction. Behind me, the thermal cycling machine works through a series of repeated heatings and coolings, with each step of the cycle hopefully generating more and more of the short sequence of DNA that we wish to locate and identify. One short sequence out of millions of connected nucleotides that make up the DNA of the mouse we are probing. If the sequence is absent, the reaction will generate no newly synthesized DNA. If it is present, enough new DNA will be manufactured to be spotted under ultra-violet light by electophoresis separation on an agarose gel containing a DNA-binding UV-fluorescing dye, ethidium bromide.

The absence or presence of these freshly-generated DNA sequences will tell me if the mouse contains a gene. In some cases, I want the gene. In others I do not. As each mouse's genetic make-up is ascertained, I will make the decision on which animal to breed with another to give me more mice with the gene I want. The breeders will be the lucky ones – those animals lacking what I need will be sacrificed.

In such ways, I directing an entire population of living animals. No natural selection here; I am the force that drives the destiny of these mice.

It is not a comfortable position. Even though every animal I select helps me move further to an understanding of muscular dystrophy, the sense of life lost never leaves me. More personal than the lives lost by farm animals to put meat on my plate, both in my knowledge of the mice and the life and death decisions I make and carry out every day.

I balance the reward with the loss, but life is precious. My heavy hand in its generation and destruction always weighs on me.