Secondary transplantation potential of Id3^−/− marrow. Graft expansion (A) and frequency (B) were evaluated for matched wild-type (▩) and Id3^−/− (▧) WBM by following the secondary engraftment assay protocol described in Figure 3. Total donor-type cells engrafting in myeloid (“M”), B-cell (“B”), and thymocyte (“T”) lineages at 5 (left) or 12 (right) weeks after secondary transplantation are given in panel A. Numbers are averaged from 3 independent transplantation experiments where a total of 32 hosts received wild-type and 34 received Id3^−/− secondary grafts. Error bars indicate SEM. (C) Thymic engraftment frequency as a function of increasing secondary marrow transplantation doses (x-axes) at either 5 or 12 weeks after secondary transplantation. (D) Thymocyte expansion differences between Id3^−/− and wild-type secondary grafts as a function of secondary marrow doses at 5 or 12 weeks after secondary transplantation. y-axes give numbers of CD4⁺ and/or CD8⁺ donor-derived thymocytes for each of the indicated secondary marrow transplantation doses. Error bars indicate SEM. Each bar represents the average of at least 3 secondary hosts. Significance was determined using the Student t test (P).