Home Again Pet Recovery System

Home Again Pet Recovery System, manufactured by Schering-Plough, is one of the two main electronic pet tagging systems used in the US (the other being AVID). Much of the rest of the world uses similar chips based on ISO 11784 & 11785, instead.

A 125khz RFID transceiver with a unique identification code is implanted between the shoulder blades of the pet. As with similar pet identification systems, a code is stored in the implanted chip that can be looked up in the HomeAgain database to determine the contact information of the pet's owner.

Criticism
See Cross compatibility of animal microchip tags