PCR quantitation of transplant engraftment. Competitive PCR amplification of Fv-wild-type (Fv+) and null (Fv–) alleles was performed as described in “Materials and methods.” The Fv+ gene product is 630 bp, and the Fv– product is 470 bp. Numbers for the standard curve (A) are the molar percentage of Fv+ template. (B) Approximately equal amounts of Fv+ and Fv– product are seen in a heterozygote (Fv+/–) DNA sample, as expected, with no Fv– allele detected in recipients of Fv-wild-type (Fv+/+) FLCs. (C) Only a faint signal from residual host Fv+ cells (1.1%-5.7%) is evident in each of the 10 Fv-null FLC recipients (J-S).