See figure 21-17 in Becker. This means you can get tissue specific probes from a cDNA library. It could be only introns, but it could be something else. Maybe all cells transcribe all genes, but only some RNA's are exported to the cytoplasm and the rest are degraded. See Becker, fig.
Transcription is the major control point.
Liver-specific genes not transcribed in brain and vice versa.