sentence: ( A ) Truncation of the spliceosomal protein Prp45 [ 2 ] or deletions of components of the cytoplasmic 5 ’ -3 ’ RNA degradation pathways , and combinations of these mutations affect mRNA levels of RPL22A , RPL22B , ECM33 and COF1 genes only to a limited extent ( with the exception of xrn1 Δ , which lowers the mRNA level by up to 50 % in some cases ) .