[INST] Here is a database schema:
atom :
atom_id [ TEXT ] primary_key
molecule_id [ TEXT ] atom.molecule_id = molecule.molecule_id
element [ TEXT ]

bond :
bond_id [ TEXT ] primary_key
molecule_id [ TEXT ] bond.molecule_id = molecule.molecule_id
bond_type [ TEXT ]

connected :
atom_id [ TEXT ] connected.atom_id = atom.atom_id
atom_id2 [ TEXT ] connected.atom_id2 = atom.atom_id
bond_id [ TEXT ] connected.bond_id = bond.bond_id

molecule :
molecule_id [ TEXT ] primary_key
label [ TEXT ]

Please write me a SQL statement that answers the following question: What is the percentage of element lead in carcinogenic molecules? [/INST]
SELECT CAST(COUNT(DISTINCT CASE WHEN T1.element = 'pb' THEN T1.element ELSE NULL END) AS REAL) * 100 / COUNT(T1.element) FROM atom AS T1 INNER JOIN molecule AS T2 ON T1.molecule_id = T2.molecule_id WHERE T2.label = '+';