(a) Naked mole-rat cells make the culture media viscous. The viscosity of water, media or media conditioned with human skin (HSF), guinea pig skin (GP SF), mouse skin (MSF) or naked mole-rat skin (NMR SF) fibroblasts for 20 days. The NMR SF + HAase bar shows naked mole-rat conditioned media digested with HAase to specifically digest HA. The NMR SF Mut are naked mole-rat skin fibroblasts that spontaneously lost the ECI phenotype7. The NMR EF are naked mole-rat embryonic fibroblasts that do not show ECI. The experiment was repeated three times; error bars show s.d. (b) Purified HA separated on pulse-field gel. Each sample was either run intact or pre-digested with HAase. The experiment was repeated five times, using both skin and lung fibroblasts (Supplementary Figure 9), and a representative gel is shown. (c) Western blot showing the levels of HA synthases in naked mole-rat adult skin fibroblasts, naked mole-rat embryonic fibroblasts, human skin fibroblasts or mouse skin fibroblasts. (d) Conserved catalytic domain of mammalian HAS2 proteins. The top sequence is the naked mole-rat HAS2. Dots indicate amino acids identical to the naked mole-rat sequence. The two amino acid changes unique to the naked mole-rat are indicated by red boxes.