In eukaryotic cells, they are formed most often in the nucleolus. In eukaryotes, the ribosomes may be classified as either 'free' or 'bound'. Free ribosomes may be found suspended in the cytosol whereas bound ribosomes are attached to endoplasmic reticulum (as such called rough endoplasmic reticulum). Free ribosomes are involved in the synthesis of proteins that will function in the cytosol while bound ribosomes in the synthesis of proteins that are to be exported or used within the cell membrane. The two types of ribosomes have similar function and structure, and in fact, are interchangeable.