Inclusion Criteria:

Patients with immunodeficiencies or histiocytic disorders 0-35 years of age with an
acceptable stem cell donor and disease characteristic defined by the following:

- Patients with histocytic disorders (hemophagocytic lymphohistiocytosis of any
etiology and refractory Langerhans cell histiocytosis) who do not meet eligibility
criteria for a myeloablative transplant procedure

- Patients with immunodeficiency disorders in whom residual immune function may not
require a fully myeloablative preparative regimen or patient is ineligible for
standard myeloablative preparative regimen (any form of severe combined
immunodeficiency [SCID], or other immunodeficiency with T cell defect)

- Patients with immunodeficiency disorders that have had poor outcome with
myeloablative stem cell transplants (including, but not limited to, common variable
immunodeficiency [CVID], Wiskott Aldrich Syndrome [WAS] if > 5 years of age, ataxia
telangiectasia)

- Patients with immunodeficiencies or histocytic disorders that require a second stem
cell transplant (SCT) for any reason

Exclusion Criteria:

- Karnofsky or Lansky performance score <70

- Glomerular filtration rate (GFR)<30% predicted

- Cardiac function <50% normal by echocardiogram

- Serum creatinine > 2x normal for age/weight

- Pregnant or lactating females

- Active serious infection that has not had an adequate course of therapy pre-SCT. Any
patient with acquired immune deficiency syndrome (AIDS) or AIDS-related complex (ARC)
or human immunodeficiency virus (HIV) seropositivity