To investigate ER stress in homocystinuria patients with remethylation defects, we have analysed the expression of genes involved in UPR (Grp78, PERK, XBP1, ATF4 and CHOP) and Ca2+ homeostasis (Herp and IP3R1) at protein or mRNA levels in controls and patients-derived fibroblasts. This evidence concerns the gene XBP1 and homocystinuria.