Mahāsaṅghika Vinaya
Bhikkhunī Pakiṇṇaka
Pakiṇṇaka 38
There are three reasons for which someone is no longer a bhikkhunī. Which three? She decides to break proper conduct i.e. puts on lay clothes, see pakiṇṇaka rule 29*, there are valid reasons for the sangha to expel her, or she turns into a man. To be explained as for the bhikkhus.