SNP1-(1-223) and SNP1-(1-208) were barely viable in the nam8∆ genetic background and SNP1-(1-193) was synthetically lethal with nam8∆. By contrast, SNP1-(1-223) and SNP1-(1-208) supported normal growth of mud1∆ cells at 20°–34° and slightly slowed growth at 37° (Figure 1B). The salient finding was that the more truncated SNP1-(1-193) allele was synthetically lethal in the mud1∆ strain, signifying that the Snp1 peptide 194FKPRRLGGGLGGRGY208 is critical for U1 snRNP function in vivo in the absence of Mud1. The corresponding peptide in U1-70K makes direct contacts to the SL1 loop (Kondo et al. 2015).