Raman spectra were acquired using a confocal Raman microspectroscopy (Ramascope Mark III, Renishaw Inc., Gloucestershire, United Kingdom) with a 785-nm diode laser (Innovative Photonic Solutions, Monmouth Junction, New Jersey). Each spectrum was acquired through a objective (Leica Microsystems Inc., Buffalo Grove, Illinois, N Plan) using 30-s exposure time and two accumulations with a laser power of 30 mW on the sample. Raman shift of the collected spectra is 300 to , and the spectral resolution is . Multiple spectra were acquired from each pellet. A summary of cell lines and the total number of spectra from each group is listed in Table 1. Background spectrum of the quartz slide was subtracted from each sample spectrum using the “automatic spectral subtraction function” in GRAMS/AI spectroscopy software (Thermo Fisher Scientific Inc., Waltham, Massachusetts). The resulting spectrum equals to the source sample spectrum minus a factored background. The final background factor is automatically determined by iteratively subtracting the background multiplied by various factors in the range of 0 to 2 in a step of 0.0001 until the residual reaches the tolerance of 1e-6. Fluorescence background was subtracted using an automated modified polynomial fitting method written in MATLAB (MathWorks, Natick, Massachusetts).28 To account for inherent variation in intra- and intersample absolute signal intensities, the spectra were normalized to their respective mean intensity in the fingerprint range of Raman shift 700 to (29).