It also housed part of the Etz Chaim Yeshiva, the largest yeshiva in Jerusalem. It is led by Rosh Yeshiva Rabbi Meir Stern. The principal Rosh yeshiva since 1982 is Rabbi Malkiel Kotler. He soon advanced to the highest shiur taught by the rosh yeshiva, Rabbi Ezra Attiya. A kollel is a yeshiva for married men. It was dominated by the Hasidic "rebbes" and Lithuanian rabbis and "roshei yeshiva". The certification in question is often conferred by the Rosh Yeshiva. He attended the Yeshiva of Flatbush religious day school. The shiur-based system was innovated at the Telshe yeshiva, where there were five levels. He established a yeshiva for Kabbalah study and actively recruited students. Wasserstein attended the Yeshiva of Flatbush for high school. He resigned his position at Yeshiva University after his arrest. In 1924, Rav Kook founded a yeshiva, Mercaz HaRav, in Jerusalem. Carlebach emigrated to Lithuania in 1938 where he studied at a yeshiva.