The miRNA expression in mammalian organs and carcinoma cells. Mouse and human organs analyzed by northern blotting were brain (B), liver (Li), heart (H), skeletal muscle (M), lung (Lu), kidney (K), and spleen (S). P19 and NT2/D1 were treated with RA at time 0, cells were harvested and total RNA extracted at the indicated times (days). The miRNA levels detected by northern blot hybridization are represented as fold change of radioactive signal over background, as described in Materials and methods. The intensity of the yellow scale corresponds to the intensity of miRNA expression; black indicates no signal was detected over background, either by quantification or by visual inspection of the phosphoimager image. White indicates cases that were not tested. The stringency of hybridization was not sufficient to distinguish single nucleotide mismatches; very similar expression profiles were observed for members of closely related miRNA families, and so these profiles may reflect a composite of expression profiles from each member.