Hypothesizing that Gas6/Axl autocrine signaling in tumor xenografts might over time lead to a more aggressive tumor phenotype, 15 mice were implanted with 106 cells each; when tumors reached 300 mm3 they were excised, and tumor fragments were serially passaged in mice for 3 x 50-day tumor re-growth cycles. The gene discussed is GAS6; the disease is neoplasm.