From Jackson Labs: Mice homozygous for the severe combined immune deficiency spontaneous mutation Prkdcscid, commonly referred to as scid, are characterized by an absence of functional T cells and B cells, lymphopenia, hypogammaglobulinemia, and a normal hematopoietic microenvironment. Prkdcscid mice accept allogeneic and xenogeneic grafts making them an ideal model for cell transfer experiments. In general, Prkdcscid leakiness is low on the NOD/ShiLtSz genetic background.