According to my biochemistry lecturer who was teaching us about radiation induced carcinogenesis for the advanced biochemistry course, leaving a petri dish of cell culture out in the sun (with the lid off) in Denver, Colorado will cause cell mutations, producing some cancer cells. This is because Denver is quite a high altitude city and has high ultraviolet radiation exposure from the sun. The high energy UV rays damage the DNA in the exposed cells, and if the damage inactivates the right genes (i.e. tumour suppressor genes), then the cell will have nothing to stop them from proliferating uncontrollably.
UV radiation generates thymine dimers between adjacent thymine bases in DNA, which disrupts base pairing between DNA double strands. This can give rise to mutations during DNA replication.
The moral, don’t go sunbathing in Denver…
(oh, and just in case there’s any misunderstanding: high energy UV is a carcinogen anywhere. It’s not only risky in Denver - use sunscreen if you’re exposing yourself to high levels of sunlight. But that’s just common sense.)