ICAM-1 and its receptor LFA-1 are involved in the syncytium formation induced in the co-culture of HTLV-1-positive and HTLV-1-negative human T-cell lines [507], suggesting that induction of ICAM-1 by Tax may facilitate infection of HTLV-1 from the virus-producing cells to uninfected cells. This evidence concerns the gene ICAM1 and infection.