His father and older cousins died, and are suspected by contemporary sources as having been systematically eliminated by the powerful praetorian prefect Sejanus.

Gemellus was born the son of Drusus Julius Caesar and Livilla on 10 October AD 19, the same day his uncle Germanicus died in Syria.

On his father's side, he was the grandson of emperor Tiberius, and his adoptive great-grandfather was the founder of the Roman Empire, Augustus.

[14] Nero was given the office of quaestor five years in advance and was married to Gemellus' sister Livia to combine the families of both possible successors.

[16][17] By 26, the emperor had withdrawn from politics altogether and moved to Capri, leaving the management of the empire to Sejanus who then began eliminating other members of the imperial family.

Germanicus' son Drusus was imprisoned within the dungeon under the Imperial palace on the Palatine Hill, where he starved to death not long after.

Suetonius says Tiberius promoted the rise of Sejanus to secure the succession of Gemellus, his natural grandson, at the expense of Germanicus' sons Nero and Drusus.

As there was no formal mechanism for succession of the imperial office, the only legal way in which Tiberius could promote an heir who was too young to assume the political powers of emperor was to bequeath to him his own estate upon which much of the Roman state had come to depend.

According to Suetonius, Tiberius had suspicions of Gaius but he detested Gemellus as he believed him to be the result of an adulterous affair by his mother.

He accomplished this with the aid of Praetorian prefect Macro and the consuls of 37 who agreed to nullify Tiberius' will, thereby allowing him to inherit all of the estates which otherwise would have been divided with Gemellus.

[27] His tirocinium, the public ceremony where young men donned the toga and became eligible for military service, was held in July 37, and a congiarium of 75 denarii was distributed to each citizen by Caligula.

Philo saw this as a ploy to put Gemellus under Caligula's patria potestas (power of a father), and perhaps to convince the Senate to go along with nullifying Tiberius' will.

[32] Suetonius says that Gemellus took medicine for a chronic cough and that the smell was detected, leading to accusations of taking an antidote for poison.