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Figure 4: (a) First principal component, describing the major differences between two groups: undifferentiated murine embryonic stem cells and the differentiated cells via formation of embryoid bodies. (b) Raman spectrum of reference RNA, revealing a good deal of similarity with (a)—hence, the major change to the spectrum during differentiation is related to a reduction in RNA levels. Note the strong peaks around 785 cm−1 (cytosine and uracil ring stretching), 811 cm−1 (phosphodiester bond stretching), and 1096 cm−1 (phosphodioxy group stretching). All spectra were acquired in 2 minutes (Reprinted with permission from [40]; Copyright 2004 American Chemical Society).