Most kappa light chain variable domains contain two conserved cis-prolines, one in position L8, where it terminates the first beta strand, the other in CDR L3, to which it confers the typical omega shape. cis-trans isomerisation of these prolines has to be shown to be the rate-limiting step in the folding of antibody fragments. Lambda light chain variable domains and heavy chain variable domains normally do not contain any cis-prolines, but have, relative to VL kappa, a one residue deletion in position L8. Stefania Spada introduced into defined kappa domains different L7-L9 sequences including those seen in kappa domains which naturally lack cis-Pro in this position and those seen in lambda chains. In Selective Infective Phage propagation experiments, Pro L8 reproducibly won out over the other alternatives, despite its negative effect on folding kinetics.