**2. Material and methods**

A Blast search of 3'UTR sequences in traces (Whole Genome Shotgun) of NCBI genome database was undertaken. We restricted the species sampling to 2 to 3 species for main division within Eutherians, and the outgroup was constituted by 2 Marsupials (*Macropus eugenii* and *Didelphis domestica*). The alignment of sequences was performed with Dialign software (Morgenstern, 2004), and then manually edited. The table 1 gathers the 27 analyzed sequences and their accession numbers. The repeat elements were identified with RepeatMasker site (http://www.repeatmasker.org/cgi-bin/WEBRepeatMasker). The target


sequences of mi-RNAs were detected with mirBase (www.mirbase.org/), following the papers of Griffiths-Jones (2004), Griffiths-Jones et al. (2006 and 2008), and Kozomara & Griffiths-Jones (2011).

Table 1. List of mammalian studied species

In order to visualise the most conservative sites of the 3'UTR, we used the method developed in Petit et al. (2006) and Martin et al. (2009). Briefly, the alignment of sequences, cleaned from insertions, was treated by Parsimony program of Phylip package vers. 3.69 (Felsenstein, 2004), to export the number of changes site by site. For each site, this number was divided by the number of sequences, allowing drawing the profile of site change rates.
