TGF-beta is a ubiquitously expressed cytokine that signals through the Smad protein family to regulate numerous cellular processes such as embryonic development and tumorigenesis. This signaling is negatively regulated by Ski, the mammalian homolog of the transforming protein of an avian retrovirus that induces oncogenic transformation of chicken embryo cells, and the related protein SnoN. Like Ski, SnoN functions by binding to the Smad proteins and preventing their phosphorylation, thereby inhibiting their ability to bind DNA and activate the transcription of downstream genes. SnoN is located primarily in the nucleus in cancer tissues or cells, but in the cytoplasm in normal tissues or primary epithelial cells. There are at least four alternately spliced isoforms of SnoN; SnoN antibody will recognize all isoforms (SnoN, SnoN2, SnoI, and SnoA).