P4.2 mRNA expression during mouse development. Northern blot analysis of RNAs from adult mouse tissues (A) and embryos at various embryonic ages (B) was conducted as described in the Materials and Methods. A 714-bp mouse P4.2 cDNA fragment containing the 3′ portion of the P4.2-coding region was used as the probe. The P4.2-hybridization signal was visualized by autoradiography (top). A longer exposure of the blot in (A) is shown in the middle. As the control, the same blots were stripped and rehybridized with a³²P-labeled β-actin cDNA probe (bottom).