HIV-1 infection of monocyte-derived macrophages (MDMs) results in induction of a myeloid cell specific ISG, CD169/Siglec1 (Fig. 1a and Supplementary Fig. 1a)8 whose expression is dramatically upregulated (fivefold) even upon low levels (<0.3 U ml–1) of IFN-α exposure (Supplementary Fig. 1b) in both infected and uninfected bystander MDMs. Here, IFNA2 is linked to HIV-1 infection.