Analysis of frozen, viable PBMCs obtained from 4 MKD patients in the Netherlands (P1–P4) demonstrated a clear accumulation of unprenylated Rab GTPases and unprenylated Rap1A (Figure 1A), compared to PBMCs from a healthy control and two unaffected heterozygous carriers of MVKH20N and MVKV377I variants [the latter described previously, (13)]. The gene discussed is RAB6A; the disease is mevalonic aciduria.