In addition, micronuclei (chromosome fragments that are not incorporated into the nucleus at cell division) were observed in vitro in cells (SAEC) that were incubated with carbon nanofibers (CNF), crocidolite asbestos, or SWCNTs (Kisin et al., 2011). Micronucleated pneumocytes were also observed in rats after a single intratracheal dose of MWCNTs (Muller et al., 2008). K-ras gene mutations, which have been reported in some lung tumors in humans and mice (Jackson et al., 2006), were observed in mouse lungs following inhalation exposure to SWCNTs (Shvedova et al., 2008)