Interferons are important cytokines which can induce a range of biological activities including cell growth and differentiation, immune regulation and antiviral responses. The molecular mechanisms underpinning these effects are now beginning to be understood and it is clear that the tightly coordinated action of a number of different cellular gene products is necessary. The HIN-200 family of genes were identified on the basis of their regulated expression by both type I and type II interferons. HIN-200 proteins have now been shown to positively and negatively regulate gene transcription and altered expression of these proteins can affect cell growth and survival. As these molecules may play an important role in the signaling cascade that is triggered upon binding of interferons to their cell surface receptors, it is necessary to understand the biochemical and molecular characteristics of the HIN-200 family members.