The shorter isoform of PUS1, PUS1-2, converts uridine to pseudouridine in the anticodon stem of tRNAs in the nucleus (Fernandez-Vizarra et al. 2007, Sibert et al. 2008). The longer isoform of PUS1 (PUS1-1) is present in mitochondria; a shorter isoform of PUS1 (PUS1-2) possessing a different N-terminus is present in the nucleus (Fernandez-Vizarra et al. 2007). In contrast, the yeast Saccharomyces cerevisiae has 2 genes: PUS1 which encodes the nuclear enzyme and PUS2 which encodes the mitochodrial enzyme. PUS1 and its substrates are conserved from yeast to humans. Like the yeast homologue, Pus1p, human PUS1 may also act on additional tRNAs, pre-tRNAs, and U2 snRNA. Mutations in PUS1 cause mitochondrial myopathy and sideroblastic anemia (MLASA) (Bykhovskaya et al. 2004, Fernandez-Vizarra et al. 2007).