As expected from previous studies (Liu et al., 2003), the K15 promoter was only active in the HF bulge, a well-characterized reservoir of quiescent stem cells that express K15, CD34 and a range of other markers (Figs. 1B, C, E; Trempus et al., 2003; Tumbar et al., 2004; Cotsarelis, 2006; Watt and Jensen, 2009). Here, CD34 is linked to hydrops fetalis.