They started with a stain of mouse in which the heavy chain and light chain loci had been `knocked-out” (made non-functional). They then introduced into embryos from these mice genes for heavy and light chains from humans; transgenic mice were then grown from these embryos. When exposed to antigens, these mice produce human antibodies against the antigens. A. Why was it necessary to start with Ig knock-out mice? B. The antibodies produced from the transgenic mice are `human” antibodies; how do these differ from the antibodies to the same antigen from normal mice? C. Would the transgenic mice recognize antigens as `foreign” differently than normal mice?