RT-PCR of key marker genes (involved in maintenance of pluripotency or early development) and housekeeping controls at varying medium serum concentrations. Starting size 750 cells. Marker Genes: Oct4, Rex1 and Nanog are markers of ES cell pluripotency, Beta-actin, Gapdh, Hprt and 18S rRNA are commonly used housekeeping genes; however, we have found that the first 3 vary unpredictably in expression during differentiation. Brachyury and nestin are markers of early mesodermal and neurectodermal differentiation respectively; Nestin is considered a CNS stem cell marker. Goosecoid is a marker of the Spemann organizer and gastrulation. Gata 4 is expressed in yolk sac endoderm and during heart formation. Nodal is expressed during gastrulation and is involved in anterior-posterior and visceral endodermal patterning. Kdr (a.k.a. Flk1, VegfR) is exclusively expressed in endothelial cells and defines multipotent haematopoietic stem cells. Fgf 5 is a marker of primitive ectoderm.