The multifunctional protein spectrin contains several different structural motifs, such as spectrin repeats and a SH3 domain. Both triple-helix spectrin repeats and the SH3 domain are believed to form independent structural entities. In alpha-spectrins the SH3 domain is localized to repeat 9, where it is positioned between helix B and helix C in the repeat unit. The presence of SH3 in repeat 9 decreases the thermal stability considerably of this repeat unit while another insert in helix C does not seem to affect the stability. Addition of one or two adjacent repeat units increases the thermal stability from ca 25 degrees C to 41 and 48 degrees C, respectively. Despite the differences in thermal stability, the folding properties of peptides comprising the SH3 domain only or together with one or more repeats are more or less the same.