Blue or 'type-1' copper proteins are small proteins which bind a single copper atom and which are characterized by an intense electronic absorption band near 600 nm [1,2]. The most well known members of this class of proteins are the plant chloroplastic plastocyanins, which exchange electrons with cytochrome c6, and the distantly related bacterial azurins, which exchange electrons with cytochrome c551. This family of proteins also includes all the proteins listed below (references are only provided for recently determined sequences).

• Amicyanin from bacteria such as Methylobacterium extorquens or Thiobacillus versutus that can grow on methylamine. Amicyanin appears to be an electron receptor for methylamine dehydrogenase.
• Auracyanins A and B from Chloroflexus aurantiacus [3]. These proteins can donate electrons to cytochrome c-554.
• Blue copper protein from Alcaligenes faecalis.
• Cupredoxin (CPC) from cucumber peelings [4].
• Cusacyanin (basic blue protein; plantacyanin, CBP) from cucumber.
• Halocyanin from Natrobacterium pharaonis [5], a membrane associated copper- binding protein.
• Pseudoazurin from Pseudomonas.
• Rusticyanin from Thiobacillus ferrooxidans. Rusticyanin is an electron carrier from cytochrome c-552 to the a-type oxidase [6].
• Stellacyanin from the Japanese lacquer tree.
• Umecyanin from horseradish roots.
• Allergen Ra3 from ragweed. This pollen protein is evolutionary related to the above proteins, but seems to have lost the ability to bind copper.

Although there is an appreciable amount of divergence in the sequence of all these proteins, the copper ligand sites are conserved and we have developed a pattern which includes two of the ligands: a cysteine and a histidine.