I know that different cells in a body express different genes and thus have different functionalities. I am wondering if there are genes that are expressed in all the cells of a certain organism. (or for that matter in all the cells that have that specific gene, for example all mammals.)? For example, maybe the gene that tells the cell to have a nucleus? Or some other function that is common to all the mammalian cells for example? Hope I'm on the right track with this question.