ALAD and essential thrombocythemia: Interestingly, despite that the rs1800435 variant in the amino-levulinic acid dehydratase (ALAD) gene has not shown a direct association with ET risk in two studies [178,179], this variant showed association with this risk in interaction with serum lead levels [178] and with the heme-oxygenase 2 (HMOX2) rs1051308G variant [179].