Despite its yield and several other remarkable agronomic traits, maize presents low level of quality proteins in its grain and the proportion of essential amino acids is less compared with other cereals. In an effort to increase the nutritional value of maize, scientists have identified specific loci able to modify the protein composition of the kernel. In 1964 the recessive gene opaque-2 (o2) was identified playing a major role in storage protein quality. Homozygous o2 kernels have higher levels of essential amino acids, especially lysine and tryptophan. Unfortunately, the associated soft and floury features of the o2 kernels cause low yield and susceptibility to diseases. Additional efforts were needed to restore the desirable hard endosperm in a modified opaque version named Quality Protein Maize (QPM). However, neither the mechanism nor the genetic components by which the modification of the endosperm in QPM lines occurs are well understood. Using a population of Recombinant Inbred Lines (RILs) derived from the cross between an opaque-2 line and a QPM line we have mapped the position of Quantitative Trait Loci controlling modification of endosperm and other traits.  Field trials have been held in College Station (CS) and Weslaco (WE), Texas. For the trait ‘opacity’, we found QTLs with major effects in chromosomes 3, 6 and 8 accounting together for about 50% of the observed variance. Work is still in progress to better define these loci and investigate the inheritance of the hallmark effects of the gene o2.  The characterization of the modifier genes will provide important tools for Marker Assisted Breeding to improve the nutritional value of this crop without negatively affecting other traits.