By Lawrence Hurley

WASHINGTON (Reuters) - Some U.S. business interests intend to signal support for gay marriage by signing on to two briefs due to be filed this week with the Supreme Court, according to lawyers involved in the process who argue that gay rights are good for business.

Various companies are set to join separate friend-of-the-court briefs, one expected on Wednesday in a case challenging the federal Defense of Marriage Act and one due on Thursday in a case that questions a California law that banned gay marriage.

Major companies are to urge the court to invalidate Proposition 8, the California law in question, and strike down Section 3 of the federal Defense of Marriage Act, which defines marriage as between one man and one woman.

The brief to be filed in the Proposition 8 case, a draft of which was obtained by Reuters, has been joined by such companies as Apple Inc, Nike Inc, Facebook Inc, Morgan Stanley, Intel Corp, Xerox Corp, AIG Inc and Cisco Systems Inc.

The two cases are to be argued on March 26 and 27.

In briefs already filed in support of marriage being restricted to heterosexual unions, business interests have not been represented. The U.S. Chamber of Commerce has not taken a position on the issue.

Lawyers at the Orrick, Herrington & Sutcliffe law firm, which is handling the Proposition 8 brief, said more names could be added to the list before it is filed on Thursday.

In the DOMA case, a source close to the case said a similar brief with more than 250 signatories is due to be filed with the Supreme Court on Wednesday. Continued...