Germ cell tumors (GCTs) are unique in that they exhibit diverse biological 
characteristics and pathological features. Although several in vivo GCT models 
are available, studies on GCTs are hampered because in vivo development of GCTs 
is time consuming and prevents a detailed molecular analysis of the 
transformation process. Here we developed a novel strategy to transform mouse 
testis cells in vitro. Lentivirus-mediated transfection of dominant negative 
Trp53, Myc, and activated Hras1 into a CD9-expressing testis cells caused 
tumorigenic conversion in vitro. Although these cells resembled embryonic stem 
(ES) cells, they were aneuploid and lacked Nanog expression, which is involved 
in the maintenance of the undifferentiated state in ES cells. Euploid ES-like 
cells were produced by transfecting the Yamanaka factors (Pou5f1, Myc, Klf4, and 
Sox2) into the same cell population. Although these cells expressed Nanog, they 
were distinct from ES cells in that they expressed CD44, a cancer stem cell 
antigen. Both treatments induced similar changes in the DNA methylation patterns 
in differentially methylated regions of imprinted genes. Moreover, despite the 
differences in their phenotype and karyotype, both cell types similarly produced 
mixed GCTs on transplantation, which were composed of teratomas, seminomas, and 
embryonal carcinomas. Thus, in vitro testis cell transformation facilitates an 
analysis of the GCT formation process, and our results also suggest the close 
similarity between GCT formation and reprogramming.