The susceptibility of human cancer cells to spreading FeLV-B replication provides a potentially instructive parallel with the mobilization and frequent reinsertion of L1 retrotransposons in >50% of human cancers (50); high levels were noted in lung, colon, prostate, and breast cancers, while the data sets for other tumor types showed few if any new insertions in renal carcinomas and melanomas. The gene discussed is PPIB; the disease is breast carcinoma.