Erythropoietin (EPO) is a hormone that helps the body maintain a healthy amount of red blood cells. It is produced primarily by the kidneys, although small amounts are also produced in the spleen, liver, bone marrow, lung, and brain. EPO stimulates the production of red blood cells, which are responsible for delivering oxygen to the tissues in the body. When specialized cells in the kidneys detect low blood oxygen levels, they increase the production of EPO, which then tells the bone marrow to make more red blood cells.

There is also a synthetic form of EPO, known as recombinant erythropoietin or erythropoietin-stimulating agents (ESAs), which is used to treat anemia that results from chronic kidney failure or some types of cancer. ESAs are produced pharmacologically via recombinant DNA technology in cell cultures. Healthcare providers clone the gene for EPO and give it to patients by injection to stimulate the production of more red blood cells.

EPO is also used in sports as a performance-enhancing drug, as it can boost the number of red blood cells and increase the availability of oxygen to the muscles. However, EPO doping is illegal, and raised levels of EPO in athletes indicate erythropoietin abuse.

In summary, EPO is a hormone that stimulates the production of red blood cells, and its synthetic form is used to treat anemia resulting from chronic kidney failure or some types of cancer. EPO is also used as a performance-enhancing drug in sports, but its use is illegal.