**1. Introduction**

Improvement of protein quality of maize incorporated the mutant gene called opaque-2, thus leading to the development of quality protein maize (QPM). Several natural mutants, which confer the highest lysine and tryptophan levels, had been identified in the 1960s and 1970s, i.e. opaque-2, opaque-6, opaque-7, floury-2 and floury-3 [1]. QPMs are having more quantity of lysine and tryptophan and lesser quantity of leucine and isoleucine. Baby corn, sweet corn, popcorn, waxy corn and high oil corn were targeted to develop quality protein maize [2]. QPM hybrids with different kernel colors have been developed and are released in India for their cultivation in various agroclimatic conditions. The technology involved in the production of QPM and normal maize are the same, but QPMs should be grown separately to maintain its purity.

In the QPM, recessive opaque-2 (o2) allele has been successfully utilized in the conversion breeding program for increasing the quality of protein in maize [3]. Primarily, maize varieties with o2 mutation were not chosen by farmers and consumers, because of opaque endosperm. Opaque-2 mutant is susceptible to pests and diseases, and it also undergoes grain breakage during milling [4]. Endosperm

modifier genes, which present hard endosperm in the o2 background, were developed at the International Maize and Wheat Improvement Center (CIMMYT), Mexico [5], and University of Natal, South Africa [6]. This leads to the development of nutritionally enriched hard endosperm maize, widely known as 'quality protein maize' [3].
