The ratio of progerin to mature lamin A correlates with disease severity in HGPS patients, and can be used to assess the effectiveness of therapies aimed at lessening aberrant splicing and/or progerin farnesylation. We have developed the first non-immunological method, based on a targeted MS approach and the use of isotope-labelled internal standards, that reliably quantifies the levels of wild-type lamin A and farnesylated progerin in cells from HGPS patients.