MYC and cancer: Based on the fact that tumor cells were GFP-negative, but contain the same number of copies of the GFP transgene, we hypothesize that cell masses are not teratomas originating from partially pluripotent cells, but that malignant tumors originated from GFP-positive iHepL cells that lost expression of hepatic factors (co-expressed with GFP from a bicistronic cassette) triggering the tumorigenicity of Myc [41].