anemia, acquired clonal sideroblastic anemia, and acquired reversible sideroblastic anemia. All cases involve dysfunctional heme synthesis or processing. This leads to granular deposition of iron in the mitochondria that form a ring around the nucleus of the developing red blood cell. Congenital forms often present with normocytic or microcytic anemia while acquired forms of sideroblastic anemia are often normocytic or macrocytic. Congenital sideroblastic anemia X-linked sideroblastic anemia: This is the most common congenital cause of sideroblastic anemia and involves a defect in ALAS2, which is involved in the first step of heme synthesis. Although X-linked, approximately one third of patients are women due to skewed X-inactivation (lyonizations). Autosomal recessive sideroblastic anemia involves mutations in the SLC25A38 gene. The function of this protein is not fully understood, but it is involved in mitochondrial transport of glycine. Glycine is a substrate for ALAS2 and necessary for heme synthesis. The autosomal recessive form is typically severe in presentation. Genetic syndromes: Rarely, sideroblastic anemia may be part of a congenital syndrome and present with associated findings, such as ataxia, myopathy, and pancreatic insufficiency. Acquired clonal sideroblastic anemia Clonal sideroblastic anemias fall under the broader category of myelodysplastic syndromes (MDS). Three forms exist and include refractory anemia with ringed sideroblasts (RARS), refractory anemia with ringed sideroblasts and thrombocytosis (RARS-T), and refractory cytopenia with multilineage dysplasia and ringed sideroblasts (RCMD-RS). These anemias are associated with increased risk for leukemic evolution. Acquired reversible sideroblastic anemia Causes include excessive alcohol use (the most common cause of sideroblastic anemia), pyridoxine deficiency (vitamin B6 is the cofactor in the first step of heme synthesis), lead poisoning and copper deficiency. Excess zinc can indirectly cause sideroblastic anemia by decreasing absorption and increasing excretion of copper. Antimicrobials that may lead to sideroblastic anemia include isoniazid (which interferes with pyridoxine metabolism), chloramphenicol (which, by inhibiting the synthesis of mitochondrial membrane protein, impairs mitochondrial respiration), cycloserine, and linezolid. Diagnosis Ringed sideroblasts are seen in the bone marrow. On the peripheral blood smear can be found erythrocytes with basophilic stippling (cytoplasmic granules of RNA precipitates) and Pappenheimer bodies (cytoplasmic granules of iron). The anemia is moderate to severe and dimorphic. Microscopic viewing of the red blood cells will reveal marked unequal cell size and abnormal cell shape. Basophilic stippling is marked and target cells are common. The mean cell volume is commonly decreased (i.e., a microcytic anemia), but it may also be normal or even