This study compares label-free methods for quantifying human proteins using shotgun proteomics. The authors evaluated two methods: spectral counting and peak area intensity measurements. They tested these methods using standard proteins spiked into a complex sample and found that both methods could accurately quantify protein abundance changes. Spectral counting showed higher sensitivity in detecting proteins with abundance changes, while peak area intensity measurements provided more accurate estimates of protein ratios. The study also compared these methods with gel staining intensities and found that both methods agreed well with independent measurements. The authors then applied these methods to analyze changes in protein expression in human erythroleukemia K562 cells stimulated under conditions that promote cell differentiation by mitogen-activated protein kinase pathway activation. They found that protein changes identified with p<0.1 showed good correlations with changes in mRNA expression. The study concludes that label-free methods are promising alternatives to isotopic labeling for quantifying protein abundance changes in complex samples.This study compares label-free methods for quantifying human proteins using shotgun proteomics. The authors evaluated two methods: spectral counting and peak area intensity measurements. They tested these methods using standard proteins spiked into a complex sample and found that both methods could accurately quantify protein abundance changes. Spectral counting showed higher sensitivity in detecting proteins with abundance changes, while peak area intensity measurements provided more accurate estimates of protein ratios. The study also compared these methods with gel staining intensities and found that both methods agreed well with independent measurements. The authors then applied these methods to analyze changes in protein expression in human erythroleukemia K562 cells stimulated under conditions that promote cell differentiation by mitogen-activated protein kinase pathway activation. They found that protein changes identified with p<0.1 showed good correlations with changes in mRNA expression. The study concludes that label-free methods are promising alternatives to isotopic labeling for quantifying protein abundance changes in complex samples.