In Japan, Holon Research Tower was released after Mirage Forest; however, due to a translation issue, the sets' releases were switched in English-language territories, causing the set to be released in North America and the United Kingdom just three days after its release in Japan. However, due to Pokémon Organized Play's prereleases, for the first time in the history of the TCG, the entire set was available in English before in Japanese.
The set introduces the Holon Research Tower, located in the middle of the Mirage Forest (which was introduced in EX Legend Maker, hence why it was to be released before Delta Species); it is the home of scientists who conduct research on Mew and strange new Pokémon. These Pokémon have been altered by a wave of electromagnetic radiation produced by an experiment gone awry at Holon labs. The result is a new species of Pokémon called the Delta Species, symbolized by the lower-case delta (δ). These new Pokémon possess types different from their original and some, in their fully evolved forms, are partially Metal-types. They could be considered a counterpart to Dark Pokémon in the EX series. EX Delta Species also introduces Holon's Pokémon, which are unique as they can double as Special Energy cards, and new Trainer cards which centralize themselves around the Holon Tower community.
Similar to cards belonging to Team Magma, Team Aqua and Team Rocket, the card layout was visually enhanced for δ Delta Species Pokémon: -
- δ Delta Species Pokémon have a character window with a "double helix" effect.
- They have "δ Delta Species" next to the character name.
- They sport a "δ" symbol where the evolution stage is situated, with the evolution window being part of the "δ" in Stage 1 and Stage 2 Pokémon.
Trivia
- This set contains only 3 Pokémon-ex, a record low for any set in the EX series.
- The holographic rare design changed exclusively for δ Delta Species Pokémon, with the card border and Pokémon itself featuring a plain holographic treatment.
- The reverse-holographic design for this set is similar to that of the new holographic rare treatment, with same foil used but only on the background image. In addition to the background, name text and rarity symbol, the reverse-holographic versions of the holographic rares now have a holographic border and HP text. All reverse-holographics feature the EX Delta Species set logo.
- The secret box topper card for this set is Azumarill.
- The prerelease card for this set is Metang δ.
