Cells from diseased mice could be grown ex vivo as EPO-dependent cell lines but did not induce the disease upon transplantation, indicating that ectopic Spi-1 expression blocks erythroid differentiation but does not overcome growth factor requirement for survival.50 Under hypertransfusion stress, tumor cells (referred as “HS-2 cells”) emerged that were able to proliferate independent of EPO and induce the disease in immunodeficient mice. This evidence concerns the gene SPI1 and neoplasm.