Demonstration of mRNA expression in transgenic mice by Northern blot. (A) RNA from wild-type mouse bone marrow (lane 1) and transgenic line F₀ no. 16 bone marrow (lane 2) and spleen (lane 3) probed with radiolabeled β-gal probe. The major expected 5-kb upper band is shown, along with a lower cross-hybridizing band of uncertain origin. In (B), the same blot is shown as in (A), after it was stripped and reprobed with control mouse β-actin cDNA probe. Several bands hybridizing to the β-actin probe are present in each lane, allowing relative quantification of the amounts of hybridizable RNA in each lane.