Subtraction screening for genes expressed in the 3PP. (A) Sagittal sections of E11.5 C57BL/6 mouse embryos were 2-color stained with anti-CD45 (green) and anti-keratin (red) antibodies. Regions of the 3PP and the 2PA are marked with blue lines. (B) Regions of 3PP (top panels, marked with dashed lines) and 2PA (bottom panels, marked with dashed lines) in histogene-stained sagittal sections of E11.5 C57BL/6 mouse embryos were microdissected for RNA preparation. Left and right panels show the sections before and after microdissection, respectively. (C) Schematic diagrams of the screening strategy for genes expressed in E11.5 3PPs.