In a type of MDS called MDS-RS, immature red blood cells (RBCs) pile up in the bone marrow. Many of these piled-up RBCs are a type of immature RBC called ring sideroblasts (RS). RS cells have a ring of small circular iron clumps around the center structure of the cell where most cells store DNA. RS contribute to having low (red) blood cell counts.
How many RS you have is part of diagnosing your type of MDS. You can be diagnosed with MDS-RS if at least 15% (15 out of every 100) of your red blood cells are ring sideroblasts.