Even small numbers of lingering cancer cells can be an ominous sign in children who have undergone seemingly suc-cessful treatment for leukemia, a study found.

Generally, children with acute lymphoblastic leukemia are considered to be in remission when doctors cannot see any cancerous cells in their bone marrow when looking through a microscope.However, a genetic-fingerprinting technique called PCR, for polymerase chain reaction, can spot as few as five cancer cells in every 100,000 cells.