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LITS (49)

Instruction:
Shade exactly four connected cells in each outlined region to form a tetromino, so that the following conditions are true: (1) All tetrominoes are connected into one large shape along their edges; (2) No 2×2 group of cells can be entirely shaded; (3) When two tetrominoes share an edge, they must not be of the same shape, regardless of rotations or reflections. (Not all four letters have to be present in the grid; for example, it is possible for your solution to not have any "I" shapes.) Cells with an 'X' (if given) are not part of any region.
Regions with fewer than four cells will not have any shaded cells.

Reference: WPF (1)

Examples:
WPF Puzzle GP 2014 - Round 1 (87)
WPF Puzzle GP 2015 - Round 2 (81)
WPF Puzzle GP 2015 - Round 5 (83)
WPF Puzzle GP 2015 - Round 7 (85)
WPF Puzzle GP 2018 - Round 8 (66)
WPF Puzzle GP 2020 - Round 5 (144)
WPF Puzzle GP 2021 - Round 6 (170)
Child puzzles:
Inverse LITSO
LITS (Borders)
LITS+
Related puzzles:
Easy as LITS
LITS (Tapa)
Pentomino Areas