Secretary of Labor Hilda L. Solis resigned her post Wednesday, offering no specific reason but that she "enjoyed my first opportunity in years to reflect on the past and my future" over the holidays.

"Leaving the department is one of the most difficult decisions I have ever made, because I have taken our mission to heart," she wrote in a letter to colleagues obtained by CNN. "As the daughter of parents who worked in factories, paid their union dues and achieved their goal of a middle class life, and as the first Latina to head a major federal agency, it has been an incredible honor to serve."

Solis provided no date when she would step down, but Labor Department officials told CNN her departure is expected around the inauguration, which is later this month.

"Over her long career in public service – as an advocate for environmental justice in California, state legislator, member of Congress and Secretary of Labor – Hilda Solis has been a tireless champion for working families," President Barack Obama wrote in a statement. "Over the last four years, Secretary Solis has been a critical member of my economic team as we have worked to recover from the worst economic downturn since the Great Depression and strengthen the economy for the middle class.

"Her efforts have helped train workers for the jobs of the future, protect workers' health and safety and put millions of Americans back to work. I am grateful to Secretary Solis for her steadfast commitment and service not only to the Administration, but on behalf of the American people. I wish her all the best in her future endeavors."

...A White House official said Attorney General Eric Holder, Health and Human Services Secretary Kathleen Sebelius and Veterans Affairs Secretary Eric Shinseki are expected to stay in their current posts. The official did not give a timetable on how long the three might stay, but none of them is expected to depart in the near future.