def break_chains(structure, m_id=0):
    """
    Break the specified structure into its chains.
    
    In case the specified structure has multiple models, only the first one is returned.

    Parameters
    ----------
    structure : Bio.PDB.Structure
        The structure to be broken into its chaines.
    m_id : int, optional
        The model ID. The default is 0.

    Returns
    -------
    structures_dict : dict of str -> list of Bio.PDB.Residue
        The dictionary mapping the chain ID to the residues belonging to that chain.
    """
    structures_dict = {}
    for model in structure:
        if model.get_id() == m_id:
            for chain in model:
                ch_id = chain.get_id().upper()
                if ch_id not in structures_dict:
                    structures_dict[ch_id] = []
                for residue in chain:
                    structures_dict[ch_id].append(residue)
    return structures_dict