2004. The elastic response of the empty prohead of the bacteriophage φ29 during nano-indentation with an AFM tip was reported [43]. A linear response was obtained when the AFM tip was pushed into the capsid; for indentation <30% of capsid height the linear regime was found to be reversible. Higher indentations caused a fracture of the viral shell and a rapid force decline. A bimodal distribution of elastic constants was found with peaks at 0.18 N/m and 0.3 N/m. The maximum forces before fracture occurred were measured at 2.5 nN.