A CpG island finder auto-detects regions of genomic DNA where 5′-CG-3′ dinucleotides are unusually clustered (often unmethylated, marking active promoters). Uses Obs/Exp = (N_CpG × L) / (N_C × N_G) via 200 bp sliding window scan with gap < 100 bp merging and two-way trimming. Toggle between Gardiner-Garden 1987 (200 bp / 50% GC / Obs-Exp > 0.6) and Takai-Jones 2002 (500 bp / 55% / 0.65) modes.

(1) Identify housekeeping-gene promoters (constitutively active regions) (2) Pinpoint cancer hypermethylation candidates (CpG island silencing) (3) Visualize regions for bisulfite sequencing / MSP primer design (4) Filter Alu-repeat false positives via Takai-Jones mode

① Paste DNA sequence (A/T/G/C, ≥200 bp recommended) ② Pick a mode: • Gardiner-Garden 1987 (default, general) • Takai-Jones 2002 (human genome / Alu filter) ③ Gap-merge slider (default 100 bp) ④ Click a Load Example: GAPDH promoter (≥1 island) or HBB promoter (0 islands) ⑤ Output: • Total GC% + sequence length • Island table (start / end / length / GC% / Obs-Exp / CpG count) • SVG horizontal track with island highlight blocks • Hover popup (start / end / GC% / Obs-Exp)

Example 1) GAPDH promoter → Gardiner mode → 1 island detected → GC% ≈ 78%, Obs/Exp ≈ 0.92 Example 2) HBB promoter → Gardiner mode → 0 islands → GC% ≈ 48%, Obs/Exp below threshold Example 3) Takai-Jones mode → re-scan with stricter criteria (500 bp / 55% / 0.65) → Alu false positives removed