Extinction of human insulin-like growth factor II expression in somatic cell hybrids

Insulin-like growth factor (IGF) II is a mitogenic polypeptide (1) that plays an important role in fetal and post-natal development (2). Both in human and rodents, IGF-II is a single copy gene that gives origin to a family of RNA transcripts (3-7). These heterogeneous mRNAs are originated from at least three promoters, functioning in many tissues of the rat during the embryonic and neonatal period (3,4,8). On the contrary, in adult animals, their expression is confined to the choroid plexus and to the leptomeninges (9). The human gene also utilizes three different promoters, two of them functioning in fetal liver and in most fetal tissues (5). The third one is a tissue specific promoter, active in adult liver only (6).