Under Family Code 1102(a), both spouses must sign a deed of trust that encumbers real estate held as community property.  If one spouse does not sign the deed of trust, that spouse can invalidate the deed of trust.  Here, the property was deeded to the spouses as joint tenants, but under Family Code 760 property acquired during marriage is presumed to be community property and merely having the deed say that the spouses take as joint tenants is not sufficient to overcome that presumption.  To trigger the community property presumption it is not necessary for the spouses to prove that the property was purchased with community funds.