POSEIDON is an ARPA-H program developing approaches to multi-cancer early detection. Its research aims to combine synthetic sensors and reporters with low-cost, self-administered screening kits that use breath or urine samples to detect 30 or more solid tumors at Stage I. The envisioned kits are designed for at-home use and would connect results with electronic health records and a health professional for review. ARPA-H describes work across sensor development and the hardware and software needed for a screening kit.