In fact, with the sole exception of leukemia, where IGF-I production and secretion is observed more frequently than IGF-II in cancer cells [45], seminal studies on the source of IGFs in cancer cells and bioptic tissues obtained from solid cancers have found IGF-I to be expressed and secreted mostly, if not exclusively, by the stromal component (fibroblast and other stromal cells) [46,47], with its high-affinity RTK receptor (IGF-IR) being variably expressed in both stromal and cancer cells [48]. This evidence concerns the gene IGF1 and leukemia.