There are strict regulations regarding the use of genetic engineering. Particulars can be found in the GenTG.
There are however exceptions to this law, for example humans as whole can per definition not become GMOs, and are hence not covered by this law. As soon as cells are separated from the subject, these are to however be seen as GMOs again.
This means, that each project should be carefully planned.
Since we are not lawyers, this information is to be seen as non-committal recommendation and therefore by no means legally binding. Consider contacting a lawyer and cross-checking the legal effectivity of your project if in doubt.