Isoform 1 appears to be a stable component of the Pol II(G) complex form of RNA polymerase II (Pol II). Pol II synthesizes mRNA precursors and many functional non-coding RNAs and is the central component of the basal RNA polymerase II transcription machinery. Isoform 1 may play a role in the Mediator complex-dependent regulation of transcription activation. Isoform 1 acts in vitro as a negative regulator of transcriptional activation; this repression is relieved by the Mediator complex, which restores Pol II(G) activator-dependent transcription to a level equivalent to that of Pol II. Isoform 1 is a component of the Pol II(G) complex, which contains the RNA polymerase II (Pol II) core complex subunits and POLR2M isoform 1. Pol II(G) appears to be an abundant form of Pol II. Additional isoforms seem to exist. Detected in adult an fetal brain. Detected in heart, kidney, skeletal muscle, small intestine, lung, prostate and testis. The adjacent MYZAP and POLR2M genes are part of a complex transcription unit. The respective transcripts derive from different promoters and are alternatively spliced. In human, some transcripts of the upstream promoter of MYZAP use exons of the downstream POLR2M gene. Belongs to the GRINL1 family.