Fibrosin is a lymphokine secreted by activated lymphocytes that induces fibroblast proliferation (Prakash and Robbins, 1998).Using mouse fibrosin as probe to screen peripheral blood leukocyte and placenta cDNA libraries, Prakash and Robbins (1998) cloned FBS1. The deduced 177-amino acid protein has at least 1 N-glycosylation site. Gel filtration indicated that FBS1 has an apparent molecular mass of about 25 kD. Mouse and human fibrosin share 72.5% amino acid identity. Northern blot analysis detected transcripts of 4.4 kb and 2.0 to 2.4 kb in placenta. Expression was also found in skeletal muscle, pancreas, thymus, testis, and leukocytes. Prakash and Robbins (1998) demonstrated that fibrosin secreted into the culture medium by mitogen-stimulated lymphocytes induced proliferation in primary human diploid fibroblasts.