Solter discovered mammalian genomic imprinting that causes parent-of-origin specific gene expression, with consequences for development and disease.

"Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed or not, depending on whether they are inherited from the mother or the father. Genes can also be partially imprinted. Partial imprinting occurs when alleles from both parents are differently expressed rather than complete expression and complete suppression of one parent's allele. Forms of genomic imprinting have been demonstrated in fungi, plants and animals. ...
Genomic imprinting is an inheritance process independent of the classical Mendelian inheritance. It is an epigenetic process that involves DNA methylation and histone methylation without altering the genetic sequence. These epigenetic marks are established ("imprinted") in the germline (sperm or egg cells) of the parents and are maintained through mitotic cell divisions in the somatic cells of an organism.

"Appropriate imprinting of certain genes is important for normal development. Human diseases involving genomic imprinting include Angelman syndrome, Prader–Willi syndrome and male infertility. (Wikipedia article Genomic imprinting, accessed 7-22).