Interferons (IFNs)(1-7) are a family of proteins, many of which share some sequence homology (7-14). They occur in a large variety of vertebrates from fish to man and are biological regulators of cell function. Their activities include the regulation of certain responses to disease. Unless induced, their concentration is below the detectable level in most organs and cells in culture.

Human IFNs are classified into three antigenically distinct types: a (at least eight species), f3 (at least two species), and y (number of species unknown). a and f3 IFNs can he induced in a variety of cells by various agents eg certain viruses, certain bacteria, and double-stranded RNA (dsRNA). y IFNs are induced in lymphoid cells by mitogens and by antiн gens to which the cells have been sensitized. The first intermediates in the induction are mRNAs that can be translated into IFNs in cell-free systems or Xenopus oocytes. It is this translatability of the mRNAs that has faciliн tated the isolation of the genes specifying human IFNs. Eight of the a and one of two f3 genes isolated are free of introns (7, 1 1, 1 5-18). The IFNs that are secreted by the producing cells bind to surface recepн tors of responsive cells (19, 20) and effect in these numerous, seemingly diverse biological phenomena (2-6). The first activity of IFNs to be discovered was the interference with the replication of various viruses (21, 22). This is the property for which these agents were named. It also serves as the basis of the most frequent IFN assays (1, 2).