Northern blot analysis of MDC expression in rat tissues. Total RNA was isolated from various rat tissues, fractionated on a formaldehyde agarose gel, and blotted onto nitrocellulose. The blot was hybridized sequentially with cDNA probes for MDC and β-actin. Probe removal was confirmed between each round of hybridization by autoradiography. Exposure times were 2 days for MDC and 8 hours for β-actin. The more rapidly migrating species present in heart represents hybridization of the β-actin probe to γ-actin, which is the major species of actin present in this tissue.27