COS-7 cells are a fibroblast-like cell line derived from the kidney of the African green monkey (Cercopithecus aethiops). These cells were established in the 1960s as a subclone of the original COS (CV-1 in Origin with SV40 genes) cell line, which was transformed with an origin-defective mutant of the simian virus 40 (SV40) genome. COS-7 cells retain the ability to express the large T-antigen of SV40, which aids in the replication of plasmids containing the SV40 origin of replication.

COS-7 cells are commonly used in molecular and cell biology research for various applications, including:

1. Transient expression of recombinant proteins: COS-7 cells can be transfected with plasmids encoding the gene of interest, allowing researchers to produce and study the function of specific proteins.
2. Gene function studies: COS-7 cells can be used to investigate the function of specific genes through overexpression, knockdown, or knockout techniques.
3. Viral production: Due to their ability to express the SV40 large T-antigen, COS-7 cells can be used to produce recombinant viruses, such as adenoviruses or adeno-associated viruses (AAVs), for gene therapy applications or viral vector research.
4. Protein-protein interactions: COS-7 cells can be employed in techniques like co-immunoprecipitation or proximity ligation assays to study protein-protein interactions within a cellular context.
5. Cell signaling studies: COS-7 cells can be used to investigate various signaling pathways and the effects of different stimuli on cellular processes.

While COS-7 cells are versatile and widely used in research, it is essential to consider that they are derived from a non-human source, which may limit their relevance in some human-specific studies.