Relative expression levels of Hif1a exon I.1 and exon I.2-containing mRNA isoforms in various mouse tissues. RNase protection assays using each 50 μg total RNA and exon I.1 and I.2-specific antisense probes. A total of 2 μg total RNA was used for the β-actin control reactions. Besides the sole protected 113-bp fragment in Hepa1 cells, a second mRNA species appeared in mouse tissues probably exceeding the 5′ end of the exon I.1 probe (134 nt).