Pediatric Endocrinology, UCLA, Los Angeles, Calif., USA were GH-dependent. In a separate set of experiments focusing on serum insulin-like activity, Froesch and colleagues demonstrated that only a small component of the insulin-like activity of normal serum could be blocked by the addition of anti-insulin antibodies. The remaining activity was termed non-suppressible insulin-like activity (NSILA) and subsequently was demonstrated to contain two soluble, low molecular weight (7 kDa) peptides [2]. A third converging line of investigation arose from studies by Dulak and Temin [3] of the mitogenic nature of bovine serum, which could be substituted by a factor secreted from cultured hepatocytes and was termed multiplication-stimulating activity (MSA). In 1972, the term somatomedin was proposed to include all of the abovedescribed factors and in some publications IGF-I is still referred to as somatomedin-C [4]. Conversely, the theory underlining the regulation of IGF-I by growth hormone is generally termed the somatomedin-hypothesis [5]. However, by 1980 the chemical identity of the biological factors discussed here was recognized as being a manifestation of two peptides with high homology to pro-insulin and a definitive designation was agreed upon. Accordingly, these two peptides were renamed insulin-like growth factors (IGFs)[6]. At the same time, a family of IGF-binding proteins was identified by Hintz and colleagues [7]. Rapidly afterwards, knowledge of the biology of IGF-I and its potential clinical utility have expanded, culminating in the recent approval of IGF-I for the treatment of Primary IGF deficiency.