An organelle is a specialized structure that has one or more specific jobs to perform in the cell, much like an organ does in the body. A eukaryotic cell contains many organelles, for example, the mitochondria, endoplasmic reticulum, nucleus, Golgi apparatus, microtubules, microfilaments and centrosomes, etc. Organelle markers (markers) that refer to proteins or other markers that are specifically expressed on certain organelles are important research tools in the study of biological processes allowing assessment of distribution, subcellular localization, and co-localization of target proteins.