> ## Documentation Index
> Fetch the complete documentation index at: https://bagel.softnanolab.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Residue

> Standard amino acid object.

## Parameters

<ResponseField name="name" type="str" required>
  Type of the amino acid in one-letter format (upper-cased)
</ResponseField>

<ResponseField name="chain_ID" type="str" required>
  ID of the polymer chain this residue belongs to
</ResponseField>

<ResponseField name="index" type="int" required>
  Internal index of the residue in the chain (0 to len(chain)-1, i.e. 0-indexed)
</ResponseField>

<ResponseField name="mutable" type="bool" default="True">
  Whether the residue can be mutated
</ResponseField>

## Methods

### three\_letter\_name

String representation of amino acid in 3-letters format.

## Example

```python theme={null}
import bagel as bg

residue = bg.Residue(name='A', chain_ID='X', index=0)
three_letter = residue.three_letter_name  # 'ALA'
```