Ribonuclease protection assay of nmt55/p54nrb mRNA expression in MCF-7 cells and ER+/ER- Human Breast tumors. Total RNA (20 μg) was prepared from MCF-7 cells (lane M), ten different human breast tumors (lanes 1–10) and calf uterus tissue (lane C). The RNA was subjected to hybridization with a human nmt55/p54nrb 499 bp SacI/BglII radiolabeled probe. The total RNA/radiolabeled probe mixtures were then digested and separated as described in Materials and Methods. Lane P indicates radiolabeled probe alone, which was not subjected to RNase digestion. Lane R represents radiolabeled probe alone that was digested with the RNase mixture. Estrogen receptor (ER) expression levels were determined by ligand binding assay and are indicated along the top of the figure. Estimated molecular weight is indicated in the MW lane.