Genes involved in certain biosynthetic pathways, such as amino acid synthesis, can become non-essential if one or more amino acids are supplied by another organism.[64] This is the main reason why many parasitic or endosymbiontic bacteria lost many genes (e.g. Chlamydia). Such genes may be essential but only present in the host organism. For instance, Chlamydia trachomatis cannot synthesize purine and pyrimidine nucleotides de novo, so these bacteria are dependent on the nucleotide biosynthetic genes of the host.[65]