Today, Sheik Sinan Abu Lohoum, the head of the Baqeel — the largest tribe in Yemen — announced his support for Yemenese protesters, asking president Ali Abdullah Saleh to step down immediately. From the AP:

The decision by the widely respected Sheik Sinan Abu Lohoum, 80, was announced in a statement issued from the United States, where he is receiving medical treatment. It was read to protesters gathered at a central Sanaa square that has become the epicenter of the protests.
Members of Abu Lohoum’s immediate family confirmed the authenticity of the statement.
Abu Lohoum’s Baqeel tribe is the larger of two that follow the Zaidi offshoot of Shiite Islam. The other — Saleh’s own Hashid tribe — has already backed the opposition.
Several senior military commanders, lawmakers, Cabinet ministers, diplomats and provincial governors have also defected to the opposition over the last week.
Saleh has repeatedly sought to appease the protesters, to no avail.
Over the past month, he has offered not to run again when his current term ends in 2013, then offered this week to step down by the end of the year and open a dialogue with the leaders of the demonstrators.