A cell type is a classification used to distinguish between morphologically or phenotypically distinct cell forms within a species. A multicellular organism may contain a number of widely differing and specialised cell types, such as muscle cells and skin cells in humans, that differ both in appearance and function yet are genetically identical. Cells are able to be of the same genotype, but different cell type due to the differential regulation of the genes they contain. Classification of a specific cell type is often done through the use of microscopy and (such as those from the cluster of differentiation family that are commonly used for this purpose in immunology).