Alongside the increase in kidney erythropoietin (Epo) mRNA expression, which denotes the physiological response to acute anaemia, mRNA levels of HMHA1 and CA9 in the xenografted tumours were also elevated in the phenylhydrazine-treated mice, indicating that low oxygen availability can upregulate HMHA1 expression in tumours (Fig. 1d, Supplementary Fig. S1B). Here, EPO is linked to anemia (phenotype).