# Off-target records

A guide's `_offset` is the uncompressed byte position of its line in `crisprDetails.tab`, resolved through
the `.gzi` index. Each line is two columns:

```text
_mismatchCounts   _crisprOfftargets
0,2,0,0,2         chr2;182255206-;466;117|chr15;101948968-;344;94|chr19;83578+;344;94|chr10;98762096+;172;21
```

- **`_mismatchCounts`** off-target counts by mismatch bucket `[0,1,2,3,4]` over the 23-nt guide; the
  on-target site is not counted. Raw absolute totals, unaffected by chunking.
- **`_crisprOfftargets`** `|`-separated. Each entry is `chrom;pos±;enCas12a_cfd;asCas12a_cfd`, the two CFD
  scores x1000. An off-target is listed only if its CFD clears `--ucscgb_cfd_threshold`; the list is
  capped at `--ucscgb_list_cap`. (So counts can exceed the number listed.)

## Coordinate geometry

`pos` is the 0-based **leftmost** base of the 27-nt window (PAM + spacer); `±` is the strand the guide
binds. The browser fetches `[pos, pos+27)`, reverse-complements it on `-`, then reads the first 4 nt as
the PAM and the next 23 as the match, the same layout as the on-target row.

## `_offset = 0` and uniqueness

`_offset = 0` means **no off-target line is stored** and the browser shows *"No off-targets found."* Two
distinct cases produce it:

| Case | `unique_TTTV` | Why no list |
|------|---------------|-------------|
| Non-unique guide | `False` | an identical protospacer exists elsewhere; off-target analysis is skipped |
| Unique, perfectly specific | `True` | specificity `1.0`, nothing cleared the CFD floor |

So **uniqueness is read from the `unique_TTTV` / `unique_TTTN` flags, not from `_offset`.** Example
non-unique row: `TTTATTTTTTTTTTGTAGAGATG`, `unique_TTTV = False`, `_offset = 0`.