For this reason, we decided to use its expression as readout of the activity of the SON network by using an ES cell line with an insertion of GFP into the Nanog locus (TNGA [25]; see Materials and Methods) and also an embryonal carcinoma (EC) P19 cell line bearing a Nanog promoter fusion to YFP (P19 OTOY cells; see Materials and Methods). Here, NANOG is linked to embryonal carcinoma.