Decreased B-cell proliferation in Ts65Dn mice. CFSE labelled, total splenocytes from Ts65Dn mice (closed bars) and euploid mice (open bars) were stimulated with (a,b) indicated concentrations of anti-IgM in the absence or presence of 10 ng/ml interleukin-4 (IL-4) or (c,d) graded concentrations of lipopolysaccharide (LPS). The data represent the percentage of cells that had undergone at least one division as indicated by CFSE analysis after (a,c) 48 hr or (b,d) 72 hr incubation (n = 4; *P < 0·05 by paired t-test).