Northern hybridization to the R72 cDNA. Electroblots of rat brain RNA (10 μg total RNA), rat kidney RNA [1 μg of poly(A) RNA], and rat liver RNA [1 μg of poly(A) RNA] were hybridized to a radiolabeled R72 cDNA probe. The same three transcripts of approximately 3.9, 3.0, and 2.8 kb in length are observed in both NAALADase-positive tissues (brain and kidney) whereas no hybridization is seen to RNA from the liver, which is NAALADase-negative. Hybridization to the poly(A) RNA (kidney) shows that the 3.9-kb band does not represent artifactual hybridization to 28S ribosomal RNA.