1. A small roughly spherical cell produced by the abnormal division of a rod-shaped bacterium that contains no chromosomal DNA and is unable to grow or divide. It is used experimentally to express nucleic acids and proteins encoded by DNA introduced to the cell, e.g. in the form of a cloning vector.

2. (microcell) An experimentally produced body consisting of a nucleus surrounded by a thin layer of cytoplasm and a plasma membrane and containing variable numbers of chromosomes. It is used as a vector for transferring one or several chromosomes into a normal cell following cell fusion.