Star Birthmark

The Joestar Birthmark (ジョースター母斑) is a distinct, five-pointed, star-shaped mark appearing in the region of the back of the left shoulder or close to the neck on all JoJos or individuals of or in certain ways connected to the Joestar bloodline.

Introduced in Stardust Crusaders, having stolen Jonathan Joestar's body, DIO inherits his birthmark. Fathered after this theft, all of DIO's sons also bear the mark, including Giorno Giovanna.

In Stone Ocean, after fusing with the Green Baby, Enrico Pucci and Weather Report also receive the mark, as the homunculus partially originated with DIO's Bone; and as twin brothers, they connect and affect one another in the same way DIO does Joseph, Holy and Jotaro in Stardust Crusaders. Once Emporio Alniño enters the new universe, he discovers that Irene possesses the mark, helping him identify Jolyne as surviving in a new form.

Albeit they haven't been depicted visibly with the birthmark, it may be assumed that George Joestar I, George Joestar II, Donatello Versus, Nicholas Joestar, and Johnny Joestar all have the birthmark as well, as they are JoJos, Joestars, or members of DIO's progeny.

In the new universe, more specifically JoJolion, the protagonist's birthmark factors in his Stand ability, and it is eventually revealed that Holy Joestar-Kira, Yoshikage Kira, Kyo Nijimura, and Josefumi Kujo also have the birthmark.

Trivia

 * Johnny Joestar is the only JoJo thus far that has not displayed a mark.
 * Josefumi Kujo is at this point the only character who had a birthmark before being seemingly connected to the Joestar family. This implies he might actually be a Joestar, but there's no hard proof for it, as he doesn't appear on the Joestar family tree in Jojolion.