Monoclonal cells are defined as a group of cells produced from a single ancestral cell by repeated cellular replication. Thus they can be said to form a single "clone". The process of replication can occur in vivo, or may be stimulated in vitro for laboratory manipulations. The use of the term typically implies that there is some method to distinguish between the cells of the original population from which the single ancestral cell is derived, such as a random genetic alteration, which is inherited by the progeny.
The most common usages of this term are:
- Monoclonal antibodies: A single hybridoma cell, which by chance includes the appropriate V(D)J recombination to produce the desired antibody, is cloned to produce a large population of identical cells. In informal laboratory jargon the monoclonal antibodies isolated from cell culture supernatants of these hybridoma clones (hybridoma "lines") are simply called "monoclonals".
- Monoclonal neoplasms (tumors): A single aberrant cell which has undergone an oncogenic event reproduces itself into a large cancerous mass.