ESI-MS spectra of MUP isoforms in urine samples according to sex and strain. Black lines show average for (a) male B6 (n = 5), (b) female B6 (n = 8), (c) male BALB/c (n = 5), (d) female BALB/c (n = 7), and circles show individual values for the relative intensity of each major peak (expressed relative to the base peak, the highest peak in each spectrum, which is set to 1). The mass spectra for a male and female S5 are shown shaded in gray in (c,d), respectively. A duplicate analysis on male and female S5 mice, non-sibling to those above, produced identical results within the boundaries of measurement error. Black arrowheads on the x-axis indicate predicted masses from the B6 genome analysis; unfilled arrowheads additional masses from the S7 genome analysis. Gray arrows above the x-axis indicate known +98 Da adducts of major mass peaks. No consistent peaks were detectable in the range 18,900-19,200 Da (Additional data file 4). The spectra for each individual sample from B6, 129 and BALB/c mice are shown in Additional data file 5.