On both of my tires (conti 0101442 or 0100175, can't remember), I got a fissure running almost uninterrupted all the circumference. It's not in the middle, but rather shifted to the left, on both of them.
They are two or three years old. The bicycle is stored on a wheel hook, tires do not touch anything. The shelter is fume-free.
Edit: Thanks for all the replies. They reminded me that the tires were sold folded several times. I'm not saying that it led to the defect, though. I tended to inflate to the upper pressure limit (and never bothered to compensate for afternoons warmer than mornings etc)