Humans cells can be genetically engineered to produce a squid protein that changes how transparent they are, which could lead to see-through patches of tissue
Source:
https://www.newscientist.com/article/2245026-genetically-altered-human-cells-can-vary-their-transparency-like-squid/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home