Warning: This page may contain spoilers! Read at your own risk!
Mew Marks are the marks people get when their DNA gets merged with an animal's. Ichigo Momomiya, Minto Aizawa, Retasu Midorikawa, Bu-Ling Huang, Zakuro Fujiwara and Ryou Shirogane all have the marks. Oddly, Berry Shirayuki has had her DNA merged with two animals, but it is unknown where her mew mark is. Some fans assume that she was too close to the injection gun at the time of her DNA infusion and as a result has the DNA entered her body directly without leaving a mark.
For the most part, the marks appear to be postioned on the midline of the characters' bodies. Ryou and Ichigo may seem like special cases, but Ryou's apparent mark is located on his neck, and is apparently symmetrical, simmilar to Ichigo's which is located on her inner thigh. If either character were to stand in a standard anatomical position (feet flat, palms foreward), the mark is seen to be on the midline of the body as well.
In Mew Mew Power, most of the time, the mew marks were erased. Ichigo's was erased in the first episode, but when it appeared in the second episode, it was lower down on her leg. The reason is probably that 4kids believed them to be tattoos, and didn't wish to encourage this matter to the children, who were watching the show.
Mew Mark locations
Her mew mark is a pink heart with whiskers underneath located on the inside of her right leg.
Her mew mark, shaped like a pair of bird wings, is on her back.
Retasu's mew mark is on her chest and it is two curves that resemble a pair of fish, resembling the horoscope; Pisces.
Her mew mark is two monkey tails on her forehead.
Her mew mark is a pair of wolf ears around her navel.
Ryou's mew mark is on his neck. However, he is always hiding it with the collars on his shirts, so it is never seen.
It is unlikely that Ringo has a marking on her body.
The location of Berry's mark remains unknown. It has been speculated by fans that because of her proximity to the injection gun, her animals' DNA passed directly into her body without leaving a proper mark.
Mew Mark Appearences
Here are some pictures of the Mew Marks that were depicted in the anime.