Interleukin 10 (IL-10) was discovered in 1989 by Mosmann and colleagues as an activity produced by murine type 2 helper T cells which suppressed cytokine production by type 1 helper T cells (1). This activity, initially designated cytokine synthesis inhibitor factor (CSIF), was isolated by expression cloning by Moore and colleagues (2), who subsequently identified and isolated a human analogue of CSIF by cross hybridization (3), using as their source a human helper T-cell clone produced by Roncarolo et al. (M.-G. Roncarolo and H. Spits, unpublished)