Seleucus II Callinicus ("great victor"), Seleucus I's great-grandson, inherited Syria from his father, Antiochus II. The event was occasioned by his mother poisoning his father around 246 BC. In addition to his mother, his father had been temporarily married to Ptolemy II's daughter Berenice, who was also killed by his mother's representatives around the same time. This prompted Berenice's brother, the recently kinged Ptolemy III, to invade the Seleucid empire.
Seleucus IV Philopater, Antiochus the Great's son, ruled a little longer than his uncle Seleucus III, but, like his uncle, lost the throne when he was assassinated by his chief advisor Heliodorus. Before he was killed, he managed to send his own son, Demetrius, to Rome to secure the release of his brother, Antiochus IV, who had also been a Roman prisoner. He ruled from about 187-175 BC.