The Importin complex consists of Importin-α and Importin-β proteins which assist in transport of arginine- or serine-rich (SR) proteins across the nucleus. Importin-12, also known as Transportin-3 or Transportin-SR, is a member of the Importin-β family and functions as a nuclear transport receptor for serine/arginine-rich proteins. Through recognition of phosphorylated RS domains, Importin-12 mediates the nuclear import of several SR proteins, such as splicing factors SFRS1 and SFRS2. By regulating the nucleocytoplasmic transport of these and other SR mRNA splicing factors, Importin-12 controls their access to mRNA and, therefore, acts a transcriptional regulator.