Red blood cells, or erythrocytes, are the most abundant cell type in the blood and function in delivering oxygen to the body’s tissues. Once oxygen is collected in the lungs, it is bound through the iron-containing hemoglobin proteins in the cell. This protein is extremely abundant in erythrocytes; in mammals it constitutes about 96% of the cell dry content by weight. Mature mammalian red blood cells lack nuclei and organelles, and so do not carry out DNA, RNA, or protein synthesis. This interesting feature keeps viruses from evolving to target these cells. In human adults, there are typically 20-30 trillion red blood cells circulating at any given time, with a life of about 100 to 120 days. They are produced continually in the bone marrow of large bones in a process called erythropoiesis. Red blood cells are frequently used in diagnostics and tests, such as in the RBC count, hematocrit, and erythrocyte sedimentation rate.