2 han (open)
Honiisou 「混一色」 is a standard yaku. Normally translated as a "half flush", this yaku features a hand composed only of honour tiles and tiles of a single suit. If the hand contains no honours, then the chinitsu yaku is counted instead. This yaku is usually referred to as honitsu or, occasionally, honichi.
A player's discard pile may lack one particular suit. This may be an indicator of a honitsu hand being developed.
Honitsu is aligned to a single suit. Therefore, this yaku is compatible with other yaku, that follows that pattern; and multiple suited yaku do not apply, namely sanshoku and sanshoku doukou. Naturally, the "single suited" yaku of ittsu can merge with honitsu, only if the fourth tile group is of the same suit or is a set of honours. Other yaku are able to function with multiple suits; but they are also possible using single suits, namely toitoi and sanankou. The same applies for chiitoitsu. Commonly, this yaku works with yakuhai, and likewise, shousangen.
Of course, honitsu is not compatible with chinitsu, because every chinitsu hand is honitsu. Honitsu combined with tanyao or junchan would actually imply chinitsu instead, since there would be no honours.
- Honiisou in Japanese Wikipedia