Cap Analysis of Gene Expression (CAGE) is a high-throughput molecular biology technique used to identify the transcription start sites (TSSs) of genes and to measure promoter activity across the genome. It is an important tool in transcriptomics and functional genomics because it enables researchers to determine where transcription begins and how gene expression is regulated under different biological conditions. CAGE specifically targets the 5′ end of messenger RNA (mRNA), which contains a unique 7-methylguanosine cap structure found in RNA polymerase II transcripts. The fundamental principle of CAGE is based on capturing the capped 5′ ends of full-length mRNA molecules. Since only mature and intact mRNA molecules possess this cap structure, CAGE selectively enriches genuine transcription initiation sites while minimizing artifacts resulting from degraded RNA. This allows accurate mapping of promoters and the identification of active genes. The procedure begins with the isolation of total RNA from the biological sample. Messenger RNA molecules with intact 5′ caps are selected using cap-trapping technology. Reverse transcription is then carried out to generate complementary DNA (cDNA) from the captured mRNA. The cap structure serves as a marker for identifying the exact transcription start site. Short sequence tags, generally 20–30 nucleotides long, are generated from the 5′ ends of the cDNA