Microcytosis is a condition in which red blood cells are unusually small as measured by their mean corpuscular volume. Microcytosis is a condition in which red blood cells are unusually small as measured by their mean corpuscular volume. It is also known as 'microcythemia'. When associated with anemia, it is known as microcytic anemia. Microcytic anemia is not caused by reduced DNA synthesis. Thalassemia can cause microcytosis. Depending upon how the terms are being defined, thalassemia can be considered a cause of microcytic anemia, or it can be considered a cause of microcytosis but not a cause of microcytic anemia.