Variation in expression of the 18645Da major urinary protein (MUP) among laboratory and wild mice with different MUP genotypes. Electrospray ionisation mass spectra of urine samples with MUP Intensity expressed relative to the highest peak in each spectrum. Blue: male profiles; pink: female profiles; dashed lines highlight 18645Da and 18893Da. All laboratory mice examined to date exhibit strong male-biased expression of an 18645Da MUP (examples A and B). Some wild mouse MUP genotypes result in similar male-biased expression of an 18645Da MUP (C), some have no detectable expression of this mass (D and E) while some express the protein in both sexes (F). Note that the atypical 18893Da MUP (darcin) does not give a signal on electrospray ionisation mass spectrometry that reflects its abundance.