a, Methylcellulose haematopoietic c.f.u. assay. Pharmacological inhibition of nitric oxide synthesis with L-NAME reduces by 50% the WSS-mediated increase in haematopoietic c.f.u.; n=3, P=0.04. Inset shows the average distribution of colony types. b, L-NAME does not affect WSS-mediated Runx1 upregulation; n=3. c, In vivo c.f.u. assay. Exposure of developing embryos to L-NAME from E8.5 to E10.5 leads to a reduction in haematopoietic progenitors in the AGM region. D-NAME, n=30; L-NAME, n=32; P=0.0008. Inset shows average distribution of colonies per AGM. Bar graphs represent average±s.e.m. *P<0.05, **P<0.005.