CpG Island Builder
Build a sequence and see when a region becomes CpG-rich — and what methylation does there.
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Simulation
Add bases, methylate CpGs and watch density, classification and gene repression change.
Add base:14/32 bases · 4 CpG
CpG density
28.6%
Classification
CpG island
Gene repression
0%
Key lesson: One CpG can matter, but the pattern across many CpGs in one region often gives a stronger regulatory signal. A CpG island is not one CpG — it is a region enriched with many.
What this means
C and G are two of the letters of DNA. When a C is followed by a G many times in a small region, scientists call that a CpG island. These regions often sit near gene 'on switches' (promoters). Adding methyl tags to many of them at once turns the gene down.