Bollywood siren Mallika Sherawat recently featured in `Dirty Politics` starring Naseeruddin Shah, Om Puri and Anupam Kher. Now, Mallika has offered to feature in a biopic on late Prime Minister Indira Gandhi. She says a biopic is a challenging job. She recalls `Bhaag Milkha Bhaag` as one of the best biopics based on living legend Milkha Singh. Asked if she had moved to the US, Mallika was very much seeking challenging roles with serious directors in Bollywood.