The House of Romanov (also transcribed Romanoff; ) was the reigning imperial house of Russia from 1613 to 1917. They achieved prominence after the Tsarina, Anastasia Romanova, was married to the First Tsar of Russia, Ivan the Terrible. Czar Nicholas II's immediate family was executed in 1918, but there are still living descendants'. The house became boyars (the highest rank in Russian nobility) of the Grand Duchy of Moscow and later of the Tsardom of Russia under the reigning Rurik dynasty, which became extinct upon the death of Tsar Feodor I in 1598. The Time of Troubles, caused by the resulting succession crisis, saw several pretenders and imposters (False Dmitris) fight for the crown during the Polish–Muscovite War of 1605–1618. On 21 February 1613, a Zemsky Sobor elected Michael Romanov as Tsar of Russia, establishing the Romanovs as Russia's second reigning dynasty. Michael's grandson Peter I, who established the Russian Empire in 1721, transformed the country into a great power through a series of wars and reforms. The direct male line of the Romanovs ended when Empress Elizabeth of Russia died childless in 1762. As a result, her nephew Peter III, an agnatic member of the House of Holstein-Gottorp (a cadet branch of the German House of Oldenburg that reigned in Denmark), ascended to the throne and adopted his Romanov mother’s house name. Officially known as members of the House of Romanov, descendants after Elizabeth are sometimes referred to as "Holstein-Gottorp-Romanov". The abdication of Emperor Nicholas II on as a result of the February Revolution ended 304 years of Romanov rule and led to the establishing of the Russian Republic under the Russian Provisional Government in the lead-up to the Russian Civil War of 1917–1922. In 1918, Bolshevik officials executed the ex-Emperor and his family. Of the House of Romanov's 65 members, 47 survivors went into exile abroad. In 1924, Grand Duke Kirill Vladimirovich, the senior surviving male-line descendant of Alexander II of Russia by primogeniture, claimed the headship of the defunct Imperial House of Russia. Since 1991, the succession to the former Russian throne has been in dispute. Surname usage Legally, it remains unclear whether any ukase ever abolished the surname of Michael Romanov (or of his subsequent male-line descendants) after his accession to the Russian throne in 1613, although by tradition members of reigning dynasties seldom use surnames, being known instead by dynastic titles ("Tsarevich Ivan Alexeevich", "Grand