For almost every amino acid (there are 20), there are unique aARS enzymes. Separate aARS enzymes exist for cytosolic (cellular) translation, and mitochondrial translation. Cytosolic aARS are names ARS1. Mitochondrial aARS are named ARS2. The first letter of the enzyme refers to the amino acid the enzyme is unique to. For example, IARS1 is the name of the cytosolic isoleucine aARS enzyme, which is responsible for attaching the amino acid isoleucine to isoleucine-specific tRNA.