Metallothioneins have a high content of cysteine residues that bind various heavy metals. Expressed both maternally and zygotically. Expressed ubiquitously during early embryogenesis, but becomes localized to the tail region during segmentation. During early larval development, expressed in discrete areas including the pronephric region, retina and cerebellum. By zinc ions. Class I metallothioneins contain 2 metal-binding domains: four divalent ions are chelated within cluster A of the alpha domain and are coordinated via cysteinyl thiolate bridges to 11 cysteine ligands. Cluster B, the corresponding region within the beta domain, can ligate three divalent ions to 9 cysteines. Belongs to the metallothionein superfamily. Type 1 family.