 From 1539, the heir to the throne was João Manuel, Prince of Portugal, who married Joanna of Austria, Princess of Portugal, daughter of Charles V. The sole son of John III to survive childhood, Prince John, was sickly and died young (of juvenile diabetes), eighteen days before his wife gave birth to Prince Sebastian on 20 January 1554.When John III died of apoplexy in 1557, his only heir was his three-year-old grandson, Sebastian.John III's body rests in the Monastery of Jerónimos in Lisbon.