The European Commission has opened an investigation regarding a network sharing agreement between two Czech mobile phone operators, namely O2 CZ / CETIN and T-Mobile CZ. The operators O2 CZ and T-Mobile CZ serve approximately 75% of the retail mobile telecommunications market in the Czech Republic. CETIN is a new network infrastructure company within the same corporate group together with O2 CZ. O2 CZ's mobile infrastructure and wholesale business has been transferred to CETIN. The cooperation between O2 CZ / CETIN and T-Mobile CZ started in 2011. The European Commission aims to investigate whether the cooperation risks slowing down quality improvements in existing infrastructure and delaying or hindering the deployment of new technologies and services. The European Commission claims that the cooperation violates Article 101 of the Treaty on the Functioning of the EU. The press release about the investigation is available via http://europa.eu/rapid/press-release_IP-16-3539_en.htm