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.

Summary
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 Alnino 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 both the original universe and Steel Ball Run universe George Joestar I, George Joestar II, Donatello Versus, Johnny Joestar, Nicholas Joestar, George Joestar III, and Holy Joestar-Kira 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 Yoshikage Kira, Kei Nijimura, and Josefumi Kujo also have the birthmark.

Trivia

 * Both Jonathan and Joseph Joestar didn't have the Joestar birthmark until Stardust Crusaders, but it was later included in the anime.
 * Johnny Joestar is the only JoJo thus far that has not displayed a mark.
 * Josefumi Kujo is the only character who has a birthmark whose connection to the Joestar family is currently unknown, as he does not appear on the Higashikata family tree in .
 * There is an error in Hirohiko Araki JoJo Exhibition 2012 showing Lisa Lisa with the birthmark, even though Lisa Lisa came from a family different from the Joestars and therefore not biologically part of the Joestar family.