The Monastery of Myrelaion (Bodrum Camii) was once located west of the Forum of Theodosius on the Mese in Constantinople. It is an important example of  a cross-in-square plan from the Middle Byzantine era. 
The church was constructed around 920 as a palace chapel attached to the palace of the emperor Romanos I Lekapenos.  The palace was constructed on a platform created by a huge rotunda, which was the remains of a Late Roman palace. It is unclear if Romanos built or acquired this palace. To adjust the church to the height of the palace, a tall substructure was constructed for it, creating a lower level similar in plan to the upper level. 
The Myrelaion is unique because it was constructed as a mortuary chapel to house the remains of Romanos and his family members, including his sons Christopher and Constantine. This is a major break from the tradition of his predecessors who were buried at the imperial Church of the Holy Apostles (at the location of Fatih Mosque) in Constantinople. He reportedly had ancient sarcophagi brought into the building for this purpose, though unfortunately there is no evidence where they were placed. Romanos became emperor by quietly pushing aside the underage Constantine VII of the Macedonian dynasty. While he married his daughter to Constantine VII, he also aspired to found his own imperial dynasty. It seems that the lack of legitimacy led Romanos to create an alternative burial place – the Myrelaion. This in turns possibly encouraged the later Byzantine practice of founding private burial churches.
Later Romanos converted the palace and its chapel into nunnery. This church, along with the Monastery of Constantine Lips, is evidence of how the spread of monasticism went hand in hand with Byzantine political and military influence in this period. The monastery had received many imperial donations in land in Asia Minor and Greece, and its convent had several illustrious members, including Katherine (daughter of the Bulgar Tsar Ivan Vladislav) and Maria, the wife and daughter of the emperor Isaac I. By 1315 it had evidently been converted into a male monastery. During the Palaiologan period its substructure was adapted to serve as a funerary chapel. After the Ottoman conquest of Constantinople, the church was converted into a mosque late in the fifteenth century by Mesih Pasha, a member of the Palaiologan dynasty and relative of Constantine XI who converted to Islam and led the forces of Sultan Mehmet II. It then took its current name, Bodrum Camii (“Basement Mosque") from the substructure of the church and was given a mihrab (prayer niche) to reorient it to Mecca and a minaret for the call to prayer.

The Myrelaion church is a cross-in-square structure (around 10 x 17 m) and has a tripartite bema and a three bay narthex. It was constructed entirely of brick and built over a lower story so as to bring it to the same level as the palace Romanos, to which it was directly attached. The Myrelaion katholikon and the north church of the Monastery of Lips (which dates earlier) are the earliest surviving examples of the complex cross-in-square covered with dome in Constantinople. These two churches are considered to have copied the monument named Nea Ekklesia, founded by Basil I in 880 and destroyed in the 15th century.

While it can be considered as a typical example of the cross-in-square church, it is also unique in certain ways. For example, in the interior the vaults are elaborated: the dome and drum have a fluted surface, forming what is known as a “pumpkin dome,” and the cross arms of the naos are topped by groin vaults rather than by simpler barrel vaults. A series of semi-cylindrical buttresses on the external walls reflect the internal structure of the monument, and create a flowing and complex effect on its western, northern and southern side. Its small, round windows are also unusual. A corbeled walkway extended around the church on the main level, and the design of the building was remarkably open.
Its bricks and mortar formed only a portion of the final product, which was lavishly embellished. It is difficult to realize the elegance of the original since it has suffered from both fires and heavy-handed restorations. Its original columns have been replaced with stone piers. The marbles and mosaics have vanished, though some fragments were found in excavations. A fragmentary fresco can be seen in the substructure, the lower part of a panel depicting a female donor kneeling before a standing figure of the Virgin Hodegitria. The building was excavated in 1964–6 by Professor Cecil L. Striker of the University of Pennsylvania, who identified the church as the Myrelaion. The building was restored in 1965–6, along with the chapel beneath it, and it is once again serving as a mosque, while the nearby rotunda was later rebuilt as a subterranean shopping mall, with its entrance on the south side of the terrace opposite the mosque.

Also see Rotunda of Myrelaion