In my opinion, one big caveat in biology is that cell lines differ from conditions in vivo. Cell lines tend to have variable karyotypes that differ very much from our actual cells. Sometimes there’s actually a very big difference. Ultimately, an organism is a black box.
We must be careful in considering results from in vitro studies for application purposes. Possibly, referencing data collected from in vitro studies using cell lines to drive useful applications, such as in metabolic engineering or drug design, may be one of the key reasons why these experiments fail most of the time. Imagine the mathematical scale – the huge margin of error from all the layers of referencing previous in vitro work that may not reflect what’s going on inside our body.
Biology is very complex and there’s so much interplay between all the different pathways and systems. It’s difficult, if not impossible, to study something in isolation and have it hold true between what happens in the test tube and what happens inside the body.