Mapping of the endoglin transcriptional start site by primer extension. The oligonucleotide PE#2 corresponding to the 5′ end of the endoglin cDNA was hybridized to poly (A)⁺ RNA from human placenta or lung. The products of annealing served as templates for reverse transcriptase. The extension products were separated on a denaturing polyacrylamide gel alongside a Sanger sequence primed on a plasmid DNA template using the same primer as that used in the reverse transcription reaction (A). Lane 1 contains the poly (A)⁺ RNA from lung or placenta, as indicated; lane 2 contains a negative control without RNA; and lane M is a size marker band in nucleotides. Extended products are denoted by arrows. The nucleotides identified at the transcription initiation are underlined (B). L, lung; P, placenta.