Which feature of a cDNA library makes it suitable for expression profiling?

• A. They originate from expressed mRNA and lack introns
• B. They include promoter regions
• C. They copy entire genome
• D. They preserve intron sequences

Answer: (A)

Expression profiling looks at which genes are being transcribed and how much. A cDNA library captures that signal because it is made by reverse-transcribing expressed mRNA into DNA. Since mRNA comes from spliced transcripts, the resulting cDNA contains only exon sequences and no introns. This intron-free, expressed-gene content is exactly what's needed to measure and study gene expression, and it avoids the introns and regulatory regions that are present in genomic DNA. So the key feature is that the library originates from expressed mRNA and lacks introns.