Immunoglobulin framework regions that demonstrate enhanced stability in the environment intracelulcar and methods of identifying them

Framework region single chain having the general structure: NH2-VL-linker-VH-COOH; or NH2-VH-linker-VL-COOH, wherein (i) the VH framework region has the amino acid sequence J (SEQ ID No. 10..) VQLVQSGAEVKKPGASVKVSCTASGYSFTGYFLHWVRQAPGQGLEWMGRINPDSGDT IYAQKFQDRVTL TRDTSIGTVYMEL TSL TSDDTA VYYCARVPRGTYLDPWDYFDYWG QGTL VTVSS; and (ii) the VL framework region has the amino acid sequence B (SEQ ID No. 2..) EIVL TQSPSSLSASVGDRVTL TCRASQGIRNELAWYQQRPGKAPKRLlYAGSILQSG VPSRFSGSGSGTEFTL TISSLQPEDVA VYYCQQYVSLPYMFGQGTKVDIKR; or a variant thereof having an identity of 90% or higher with the region simple chain or frame while maintaining enhanced stability as determined by ensayodel Example 2.