On Tuesday, the DFB told ESPN FC it had begun proceedings. Under its rules, the club could face a fine or even a stadium closure.
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Hannover president Martin Kind has apologised for the racial abuse Mainz players Leon Balogun and Anthony Ujah were subjected to on Saturday.
Former Netherlands international Nigel De Jong signed a short-term deal with Mainz on Friday.
Borussia Dortmund picked up their first win since September in new coach Peter Stoger's first game, as they beat Mainz 2-0 on Tuesday.
Mainz goalkeeper Robin Zentner said his air-kick against Borussia Monchengladbach happened when he mistook the penalty spot for the ball.
Eintracht Frankfurt extended their unbeaten run to four Bundesliga matches as they were pegged back in a 1-1 draw against Mainz at Opel Arena.
Schalke moved up to fourth in the Bundesliga for 24 hours at least thanks to a 2-0 victory over Mainz at the Veltins Arena on Friday night.
Bayern Munich got back to winning ways in the Bundesliga after easing to a 4-0 victory over Mainz.
Mainz have announced the signing of defender Abdou Diallo from Monaco on a five-year deal for an undisclosed fee.
Huddersfield have signed Danish goalkeeper Jonas Lossl on a season-long loan from Bundesliga side Mainz.
Middlesbrough forward Viktor Fischer has joined Mainz, the Bundesliga club have announced.
Mainz have signed Romania international Alexandru Maxim from Bundesliga new boys Stuttgart on a four-year deal.
Mainz have signed former Germany international goalkeeper Rene Adler on a two-year contract.
Mainz have appointed Sandro Schwarz as their new manager.
Mainz have confirmed head physiotherapist Christopher Rohrbeck will be leaving to join Liverpool.
Martin Schmidt has left his job as Mainz coach by mutual consent, the Bundesliga club have announced.
Borussia Dortmund overcame a traumatic week to retain fourth place in the Bundesliga by toppling Eintracht Frankfurt 3-1.
Liverpool manager Klopp guided Mainz to promotion in 2004 and said: "I will never forget this day."
Mario Gomez plundered a hat trick inside seven minutes as Wolfsburg saw a remarkable fightback victory over Bayer Leverkusen snatched from their grasp
Thomas Muller ended his Bundesliga scoring drought to give champions Bayern Munich a 1-0 win at Borussia Monchengladbach.