these responsibilities very seriously and was noted for her incisive reviews and for her thoughtful and astute comments in committee meetings.

Charlotte Friend used to say that she started an industry, and it was true. In the two decades following the initial isolation of FLV, perhaps a third of those doing cancer research spent some time working on one or the other of the model systems she had provided. Even today, many aspects of the Friend virus complex (FLV/SFFV) remain unique among tumor viruses so that it continues to be an important, widely utilized model system (reviewed in ref. 10). For example, it acts as an acute transforming virus but does not contain an oncogene. It is immuno-suppressive and, therefore, a model for human immuno-deficiency virus (HIV). It stimulates abnormal erythroid hyperplasia by binding of virus (SFFV) glycoprotein to a growth factor (erythropoietin) receptor, a novel mechanism that may explain how other viruses can interfere with normal growth regulation.

Friend paved the way for a great many other avenues of research. She was the first to show, using her virus, that animals could be immunized with retrovirus preparations and protected against development of the disease.¹⁸ Her experiments indicating that such protection is possible are frequently cited by those now trying to develop a vaccine against HIV. Leukemic cells induced by Friend Virus have been used in the first demonstration that a virus-induced malignancy requires for its expression the alteration or deletion of an "anticancer" or suppressor gene. Friend was one of the first to show that cells maintained in culture can actually undergo the successive steps leading to differentiation to a specific cell type. Her system was the forerunner for the establishment of other cell culture models to study