The purpose of this trial is to determine if administration of rituximab after allogeneic
stem cell transplantation can reduce the incidence of chronic GVHD. Chronic GVHD is a medical
condition that can occur after bone marrow or stem cells are transplanted form one individual
to another. After the transplant, the donor immune system may recognize the recipient body as
foreign and may attempt to "reject" the body. Rituximab is a drug that interferes with the
immune system function by specifically targeting B cells and killing them.