Thioredoxins comprise a family of small proteins that, by catalyzing the oxidation of disulfide bonds, participate in redox reactions throughout the cell. Proteins that contain thioredoxin domains do not necessarily convey the oxidative properties of thioredoxins, but generally function as disulfide isomerases that enzymatically rearrange disulfide bonds found in various proteins. TXNDC9 (thioredoxin domain-containing protein 9), also known as APACD (ATP-binding protein associated with cell differentiation), is a 226 amino acid protein that contains one thioredoxin domain and may be involved in cell differentiation events. The gene encoding TXNDC9 maps to human chromosome 2, which houses over 1,400 genes and comprises nearly 8% of the human genome. Harlequin icthyosis, a rare and morbid skin deformity, is associated with mutations in the ABCA12 gene, while the lipid metabolic disorder sitosterolemia is associated with defects in the ABCG5 and ABCG8 genes. Additionally, an extremely rare recessive genetic disorder, Alstr鰉 syndrome, is caused by mutations in the ALMS1 gene, which maps to chromosome 2.