IGF-1R (The insulin-like growth factor 1 receptor) is a protein found on the surface of human cells. It is a transmembrane receptor that is activated by a hormone called insulin-like growth factor 1 (IGF-1) and by a related hormone called IGF-2. It belongs to the large class of tyrosine kinase receptors. The IGF-1R is implicated in several cancers, including breast, prostate, and lung cancers. In some instances its anti-apoptotic properties allow cancerous cells to resist the cytotoxic properties of chemotherapeutic drugs or radiotherapy. IGF-1 binds to at least two cell surface receptors: the IGF1 Receptor (IGFR), and the insulin receptor. Studies in female mice have shown that both supraoptic nucleus (SON) and paraventricular nucleus (PVN) lose approximately one-third of IGF-1R immunoreactive cells with normal aging. Mutations in IGF1R have been associated with craniosynostosis.