Skeletal and cardiac muscles are striated myofibers that contain highly organized sarcomeres for muscle contraction.Recent studies revealed that Smyd1,a lysine methyltransferase,plays a key role in sarcomere assembly in heart and trunk skeletal muscles.However,Smyd1 expression and function in craniofacial muscles are not known.Here,we analyze the developmental expression and function of two smyd1 paralogous genes,smydla and smydlb,in craniofacial and cardiac muscles of zebrafish embryos.Our data show that loss of smydla(smyd1amb5)or smydlb(smyd1bsa15678)has no visible effects on myogenic commitment and expression of myod and myosin heavy-chain mRNA transcripts in craniofacial muscles.However,myosin heavy-chain protein accumulation and sarcomere organization are dramatically reduced in smyd1bsa15678 single mutant,and almost completely diminish in smyd1amb5;smydlbsa15678 double mutant,but not in smyd1ambs mutant.Similar defects are also observed in cardiac muscles of smyd1bsa15678 mutant.Defective craniofacial and cardiac muscle formation is associated with an upre-gulation of hsp90α1 and unc45b mRNA expression in smyd1bsa15678 and smyd1amb5;smyd1bsa15678 mu-tants.Together,our studies indicate that Smyd1b,but not Smyd1a,plays a key role in myosin heavy-chain protein expression and sarcomere organization in craniofacial and cardiac muscles.Loss of smyd1b results in muscle-specific stress response.